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Attributes

• Strength: Whatever fighting style you chose, ranged or melee, this is probably the best way to increase damage (although it's a little pricy to ranged characters). A must for melee characters.
  
• Dexterity: Equally great to ranged characters, many ranged feats require high dexterity ratings. Note that there are some dexterity-based skills in the kensai's list. Unarmed kensai need high ratings in this attribute, too, because of two-weapon and multiweapon fighting feats' requirements.
  
• Constitution: This is absolutely needed to the kensai. Not only it gives you extra hit points, but it boosts your concentration checks, too.
  
• Intelligence: Not actually needed, but the combat expertise feat that is required to enter this class you need a score of 13.
  
• Wisdom: The Kensai has a good will save progression and only one skill is keyed off this attribute. I'd say that this is an attribute you can dump.
  
• Charisma: Two of your skills are keyed to charisma. Thing is that these are really great: diplomacy, intimidate. Since you are a melee character, demoralize optimization is going to help as it has teamwork benefits (stacking fear effects) and on the other side you gain a roleplaying edge.
  

Races

• Human: This is the best race from an optimization side. Bonus skill points and the feat help fulfilling the requirements of this prestige class.
  
• Dwarf: Bonus constitution but loss of charisma.
  
• Elf: The constitution penalty is really bad, but it can probably serve as an archer.
  
• Illumian: Technically a human and can get the shadow blade prestige class, which gives you a weapon with some cool qualities to stack with kensai.
  
• Thri-Kreen: Probably the standard race for unarmed kensai. These guys get four arms that can use as natural weapons. The main problem is the racial hit dice they get, as their level adjustment is pretty low and you can pay it off easily. Note that intelligence hit will hurt your requirements.
  

Templates

• Feral [SUP]SS[/SUP]: This template is well known in the character optimization boards as the best way to gain the pounce ability at a low level adjustment, pre-complete champion. It also nets you two claw attacks to use as signature weapons. Use it with extreme caution, as it reduces your intelligence by four.
  
• Insectile Creature [SUP]SS[/SUP]: The limbs of the insectile template cannot be used for attacks, but the fiend folio graft raking tentacle that replaces an arm or forelimb, says: [...]allowing the grafted creature to make a natural attack with the raking tentacle[...]. So if you graft all your six hands, you can probably attack with them. As a treat you get immunity to flanking, wide vision and tremorsense. The dexterity boost is useful, but the level adjustment is really bad without the option to buy it off.
  
• Anthropomorphic Animal [SUP]SS[/SUP]: This template offers two great options with loads of natural weapons: The giant octopus and squid. The first has no level adjustment, two hit dice of monstrous humanoid and buffed dexterity, but a constitution hit. The second one is superior, but that comes at the price of a single level adjustment. Monstrus humanoid comes with full base attack bonus, so to fulfill requirements you only need a class that offers those skills as class skills. Anthropomorphic characters have reduced charisma, something not so cool for kensai.
  

Player's Handbook

• Improved Disarm/Trip: Combat Expertise is a requirement feat and you receive huge bonuses to your strength. Tripping (or disarming) is a nice tactic that benefits both you and your allies, although you'll have problem with it if you are fighting unarmed.
  
• Exotic Weapon Proficiency: Some of the best double weapons are exotic. If you are gonna use one of those, this is an option. You can maybe use exotic weapon master after kensai levels, although that messes up your skills.
  
• Improved Unarmed Strike: This is a requirement if you are going to use your fists and you don't have some sort of natural weapon. Also it's a nice way to add more attacks to your routine.
  
• Skill Focus (Concentration): Too narrow. It's better to get an item that enhances concentration.
  
• Two Weapon Fighting Tree: If you are going to fight unarmed or with double weapons, you most probably will want to get this. The main problem is that you need high dexterity, so your MAD gets even worse.
  

Monster Manual

• Improved Natural Attack: Increase the damage of one form of your natural attacks. It's not that great, as your damage won't be significantly increased. Take it if you have the feats to spare or if your base damage of your natural attack is already high.
  
• Multiattack: This feat has low requirements and is great if you are going to use natural weapons as secondary attacks.
  
• Multiweapon Fighting Tree: You can't really use iterative attacks with natural weapons and it will be difficult finding a large number of weapons to use with multiple limbs without using cheese.
  

Book of Exalted Deeds

• Ancestral Weapon: Nah, don't get this, at least with your signature weapon. That's because you will put a gp cap to the cost of your weapon, when theoretically with just a level in kensai you can put as many gp enhancements on your signature weapon as you want.
  
• Vow of Poverty: Since you don't spend money to enhance your signature weapons, this is an option. The penalties of the feat are lessened as you can take some enhancements that make you able to fly, penetrate damage reduction, etc.
  

Draconomicon

• Improved Multiattack: Gain no penalty on your secondary attacks. Well essentially multiattack gives a +3 bonus to your secondary attacks (from -5 to -2) and improved multiattack gives only a +2 bonus.
  
• Rapidstrike: Make an extra attack with one of your natural weapons at a -5 penalty. Not that good, but required for improved rapidstrike.
  
• Improved Rapidstrike: This nets you four extra attacks with a pair of your natural weapons. The penalties are huge, but they are extra attacks and do not affect the rest of your attack routine.
  

Kensai Fighting Styles

kensai are weapon masters and they can use a lot of different fighting styles. However since you have an option to imbue multiple weapons, the optimized choice is to select a race with multiple natural weapons of the same type or a double weapon.

• The best choice is the multiple natural weapons, but that comes at the cost of level adjustment and/or racial hit dice.
  
• The next best choice is to get a normal race and enhance their fists. Costs only 120% for both weapons, but the two will have the same enhancements.
  
• Double melee weapons are easy to come by, so for normal kensai that want a variety of enhancements to their weapons, this choice can work.
  
• I wouldn't suggest a ranged kensai, but if you absolutely want one and races of the wild is available, get yourself and elvencraft bow. This is actually a double weapon that must be imbued separately and you can interchange melee and ranged attacks. Consult with your DM if you can imbue the shaft as a signature kensai weapon (since it is actually a quarterstaff, you may need another weapon focus or weapon focus (Elvencraft Bow).
  

Weapon Enhancements

DMG

• Anarchic: Not gonna happen. You need to be lawful in alignment.
  
• Axiomatic: Unless you know that your opponents gonna be of a specific alignment, this isn't worth it.
  
• Bane: As above, but better. Campaigns sometimes have a specific creature type as a theme, so if you identify it this is gonna work.
  
• Brilliant Energy: You cannot harm undead, constructs or objects and the enhancement bonus is huge. Only take this if you aren't going to face those creatures, but since it's permanent, you never know.
  
• Dancing: Assuming that your signature weapon is your best one, this is absolutely worthless, unless you have another weapon of great power.
  
• Defending: If you have a high amount of natural weapons, this would be great, gaining you large amounts of dodge bonuses.
  
• Disruption: The DC is crap and only works with bludgeoning weapons. Not worth it.
  
• Distance: Average property. If you use it with an elvencraft bow it may undermine you.
  
• Flaming/Frost/etc: Elemental damage is not good, especially fire which is probably the most common element resistance.
  
• Flaming/Icy/etc Burst: As above.
  
• Ghost Touch: This is absolutely essential if you are going to face incorporeal creatures.
  
• Holy/Unholy: This is an option, the good/evil theme is the most common in campaigns.
  
• Keen: I am against this, but many people will revolve :D. If you have many natural weapons and versatile unarmed strike, you will increase your critical hit percentage.
  
• Ki Focus: If you are a monk, you are better off choosing your unarmed stike as your natural weapons and skip this.
  
• Merciful: Only useful if you have vow of nonviolence probably. At any rate it's pretty much worthless.
  
• Mighty Cleaving: The cleave feat is useful probably only at low levels. Still it's a crappy feat, so this doesn't make it any better.
  
• Returning: This is an option for throwing weapons, but if you throw your weapon, remember that you get it back at the start of your next turn.
  
• Seeking: This is actually good and cheap. Helps with miss chances, so it is a nice and more useful substitute for ghost touch.
  
• Speed: This is not a good enhancement for both heads of a weapon or for your natural weapons. This is because you are getting only one extra attack.
  
• Spell Storing: This is a bomb! If you have multiple natural weapons you can even nova. It's cheap, too. This will require some spellcaster in your team to fill your weapons or some spellcasting ability on your behalf.
  
• Throwing: Nah, forget about this one. It's useless, as you can just move and attack. Heck, you can even choose a throwing weapon as your signature one and skip this.
  
• Vicious: Yuck, go for another enhancement that gives you a damage bonus and doesn't hurt you.
  
• Vorpal: Too expensive and useless. A huge number of creatures and creature types are immune to this effect, too.
  
• Wounding: Constitution damage is good, but there are many creatures immune to constitution damage or critical hits. It is highly dependable on the campaign, so choose wisely.
  

Magic Item Compendium

• Banishing: Particularly useful to those who have multiple natural weapons. If you encounter a lot of extra planar creatures, this is an option. The DC is average/good and scales. You get a lot of uses with multiple natural weapons.
  
• Binding: If you have problems dealing with hoping enemies, this is a solution. The duration and uses are good for whatever fighting style you choose.
  
• Blindsighted: If you have problems with invisible or hiding enemies, this comes handy, but it's a little expensive. The effect lasts for 1 minute, so it will serve you for the average encounter. Three times per day can be augmented by using natural weapons.
  
• Bloodfeeding: This is actually nice. With a bag of tricks and subdual damage you can charge up your weapons with blood points. Unfortunately they last for only an hour. It's sort of mini-nova for the early levels.
  
• Bloodstone: It's a way to nova, but you must keep charging your weapons with vampiric touches. You get less versatility but more damage.
  
• Collision: Increase your damage by 5. I haven't done any calculations about this, but it may be worth getting.
  
• Defensive Surge: You already have the requirement feat for this ability and a +2 bonus isn't very bad. Multiple bonuses from the same source do not stack, so it probably doesn't work with multiple natural weapons.
  
• Dispelling, Greater: Great caster level and essentially it gives you three 6th level spells at a +2 price. Handy in quite a few occasions, giving you an edge against spellcasters.
  
• Everbright: It's just a gp enhancement, so it won't be much of a problem to get this. The activated ability is pretty much crappy, but immunity to acid damage and rusting effects is interesting.
  
• Impedance: This is interesting against spellcasters, but only with a debuffer to reduce the target's skill and ability checks. Otherwise the DC is pretty much average.
  
• Implacable: This would be great for unarmed kensai if the skill check's DC was higher. A creature striking an enemy four times in a single round will eventually deal 8 * 5 = 40 more damage. Remember that not all creatures bleed though. As an added bonus, this can mess up healing magic to an affected creature and counts as adamantine for the purpose of overcoming damage reduction to aberrations.
  
• Manifester: Particularly useful to kensai with knowledge of psionic powers, this will give you some bonus power points to enhance your powers. Great for characters with lots of natural attacks, and it's a gp enhancement.
  
• Morphing: Change your weapon to fit best the current situation. While it's pretty obvious with normal weapons, it's very flavorful with natural weapons, changing your fists, bites or tentacles with axes and swords.
  
• Power Storing: As spell storing. Remember that you can have both and they require free actions to discharge.
  
• Psychic: You can invest in this enhancement if you are going to have a power point reserve of 30 or better. It can save you some points to spent on other enhancements. Note that if you use the XPH for this ability, it's even better, as it is a gp enhancement.
  
• Shadowstrike: It's only a gp enhancement that gives you bonus reach and your target is denied his dexterity bonus to AC for that attack. Remember that it's activated as a swift action, so you can't nova with this one.
  
• Shattermantle: Useful if your spellcasters have low caster level and/or can't penetrate spell resistance easily. If you have a large number of attacks, this is worth it. Also it is useful if you are using the spell storing enhancement with spells that allow SR.
  
• Sizing: This is great. The possibilities are great and it's just a gp enhancement. You can make weapons tiny to pass them from guards or enlarge shaft weapons to use them as ladders, etc. It would be interesting with natural weapons though.
  
• Transmuting: Take it so you can penetrate damage reduction without a problem.
  
• Vampiric: Pretty expensive for its effects, but it nets quite a few hit points. Not to mention that you can heal to maximum with a bag of tricks and subdual damage.
  
• Vanishing: Gain a limited range dimension door some times per day. It's only a gp enhancement, so it's easy to get. It's great if you have multiple natural weapons, netting you multiple mini-dimension doors per day.
  

Dungeon Master's Guide II

• Elemental Power: Whatever elemental you choose this is a great ability that gives you extra flavor. Large and huge are probably the best. Use it with multiple natural attacks, so that you can summon more than one elemental per day.
  
• Surge: For a number of times equal to your charisma modifier and for a low gp price, get the ability to deal extra damage as a swift action. You can have multiple of these abilities on your weapons, but holy and unholy surge are probably the best.
  
• Sudden Stunning: You already can have a nice charisma score, so this is useful and cheap. It also scales according to your level, making it generally more useful.
  

Natural weapons are weapons that are physically a part of a creature. A creature making a melee attack with a natural weapon is considered armed and does not provoke attacks of opportunity. Likewise, it threatens any space it can reach. Creatures do not receive additional attacks from a high base attack bonus when using natural weapons. The number of attacks a creature can make with its natural weapons depends on the type of the attack—generally, a creature can make one bite attack, one attack per claw or tentacle, one gore attack, one sting attack, or one slam attack (although Large creatures with arms or arm-like limbs can make a slam attack with each arm). Refer to the individual monster descriptions.

Unless otherwise noted, a natural weapon threatens a critical hit on a natural attack roll of 20.

When a creature has more than one natural weapon, one of them (or sometimes a pair or set of them) is the primary weapon. All the creature’s remaining natural weapons are secondary.

The primary weapon is given in the creature’s Attack entry, and the primary weapon or weapons is given first in the creature’s Full Attack entry. A creature’s primary natural weapon is its most effective natural attack, usually by virtue of the creature’s physiology, training, or innate talent with the weapon. An attack with a primary natural weapon uses the creature’s full attack bonus. Attacks with secondary natural weapons are less effective and are made with a -5 penalty on the attack roll, no matter how many there are. (Creatures with the Multiattack feat take only a -2 penalty on secondary attacks.) This penalty applies even when the creature makes a single attack with the secondary weapon as part of the attack action or as an attack of opportunity.

Natural weapons have types just as other weapons do. The most common are summarized below.

• Bite: The creature attacks with its mouth, dealing piercing, slashing, and bludgeoning damage.
  
• Claw or Talon: The creature rips with a sharp appendage, dealing piercing and slashing damage.
  
• Gore: The creature spears the opponent with an antler, horn, or similar appendage, dealing piercing damage.
  
• Slap or Slam: The creature batters opponents with an appendage, dealing bludgeoning damage.
  
• Sting: The creature stabs with a stinger, dealing piercing damage. Sting attacks usually deal damage from poison in addition to hit point damage.
  
• Tentacle: The creature flails at opponents with a powerful tentacle, dealing bludgeoning (and sometimes slashing) damage.
  

Unarmed Strikes and Natural Weapons

Just as a creature can add weapon attacks to a full attack made with natural weapons, so too can it combine unarmed attacks with natural weapons. Two options are available to accomplish this task.

A creature can choose to treat its unarmed attacks as its primary attacks and its natural weapons as secondary attacks. (This method is normally used to add weapon attacks to a natural attack routine.) The creature must make all unarmed attacks with its primary limb, which prevents that hand from being used for a natural attack such as a claw or slam. It uses its full base attack bonus for the natural attack, gaining additional attacks as normal for a high base attack bonus, and adds its full Strength bonus on damage rolls. Of course, each of these attacks provokes an attack of opportunity if the target is unarmed (unless the creature has Improved Unarmed Strike). However, its natural weapons all become secondary attacks, taking the -5 penalty on attack rolls (or -2 with the Multiattack feat) and adding only half the monster's Strength bonus on damage rolls.

A simpler method is to treat the creature's unarmed attack as an off-hand attack. (After all, an unarmed strike is rarely as effective as a weapon attack would be, so it doesn't really merit the same level of priority in the average monster's attack array.) Instead of using its primary limb to deliver the unarmed attack, it uses a kick, head butt, or other appendage that isn't otherwise used to deliver a natural attack. The creature gains one unarmed strike, which deals damage appropriate to its size plus half its Strength bonus (since it's an off-hand attack). A creature using this method suffers a -4 penalty on all attacks (since it's effectively fighting with two weapons and its off-hand weapon is light). The damage for its natural attacks is unchanged. This method requires fewer calculations on the fly, so it's probably easier to use in play.

And, are you doing a writeup on classes to enter from? If so, be sure to mention that samurai and kensai DO NOT mesh at all. It may sound from their names as if they do, but they don't. And you might want to mention it's from Complete Warrior, you'll probably do that in the first post with a short writeup of the class.