way back in the 3rd ed days i had some kind of flying half demon guy who would swoop down from trees in the forest and rob people. i cant remember what specific race or class he was but it was fun for a mini weekend game.

these days my characters are pretty grounded in reality.

  After playing for 30 years, I could write a book on odd characters.

My weirdest 4E character is my eladrin feylock mc swordmage Harrow...

(Harrow looks somewhat like a dark non-glam version of David Bowie in Labrynth, but the character is heavily inspired by slasher films and things like that.)

Harrow didn't really sign a Pact, per se.
Harrow is his Pact.
And technically his own Patron, as well.

    An orphaned eladrin raised by humans who had no idea of his fey heritage, the man who would become Harrow was shunned by the folk of his town for his strange appearance. With no knowledge of his true race, Harrow was slowly driven mad by visions of the Feywild as a child, and by teleporting through it when his fey step ability began to develop. He often found himself vanishing in response to stress or suddenly finding himself in a place he was just thinking about, and would regularly scare others by showing up behind them without warning.
   A borderline psychotic, he was unable to deal with the day-to-day details of earning a living in his small community and lived in a ramshackle hut on the edge of town. Many rumors were whispered about him, including the possibility that he was possessed by demons.
   Eventually leaving his village to become a wandering madman, Harrow was always one step ahead of the consequences of the last time he lost his temper or his mind.

    One day, in a foul mood after wildly attacking a drunkard who'd spilled ale on him and being thrown out of yet another tavern for his outburst, Harrow was travelling through a forest on his way to the next town when a small Tinkerbelle-style faerie flew up to him and introduced herself as Alma(something)dorabella(something,something)-Hannabee-Athella(something)archemediahermesia-hera...something something.... (Her name is over thirty seven syllables long).

By the time that "Alma" (or "that damned mosquito", as Harrow calls her) had finished reciting her whole name...
and asking him in a very perky and chipper voice why he wasn't smiling (because everybody knows that smiling makes everything better) and happy...
Harrow was so far into a teeth-gnashing, about-to-bleed-from-the-eyes black rage...

That he grabbed her out of the air and ATE her.

Alma, being an immortal fey, didn't exactly die - she's now inside Harrow, who drains off her ever-renewing magic to power his abilities.
Being immortal, but digested and dispersed throughout Harrow's body, Alma's body regrew into a new form within him, forming something almost like a second central nervous system. She's now just as dependent on Harrow's lifeforce as he is so they currently exist as a symbiotic being, Harrow unable to survive the loss of Alma's power, Alma unable to exist in the real world without Harrow's physical form.

Alma, previously an obliviously happy, cheerful, and annoyingly outgoing fey, has been driven mind-rippingly insane by the whole process, and now constantly projects her insane ravings, emotional outbursts, and scattered thoughts into Harrow's mind like a "little voice in the back of his head" on steroids...

Harrow had always been rather dark and nasty, and one more messed-up voice in his head didn't improve anything. He and Alma have a very co-dependent and dysfunctional relationship, and he has a tendency to fall into screaming, foaming fits, rolling on the ground and babbling in multiple voices while the two of them argue in his mind.

 All of Harrow's powers are flavored as either horror movie special effects or as a glowing green image of Alma coming forth and doing things.

I think I just put together my weirdest character... My favorite of the cursed floating spheres thread, the revenant shaman|ranger-in-a-box. Physically, he is a revenant inside a 3' spherical shell of 1/4" magically strengthened steel with the appearance of ruddy bronze. He summons his spirit companion, fluffed to be his own soul, outside of the shell and projects all of his ranged and area attacks from it. His shell is carried on a Tenser's Floating Disk with Tenser's Circular Shield, and must drag his body along with him or else he will vanish outside of its range.

An angry corpse, trapped inside a shell he cannot escape, his spirit  lugging around his coffin because it's tethered to his physical body. I  imagine him having dug his own grave up, getting frustrated at vanishing  every time he was outside of 20 squares of his body, deciding to drag  the damn thing around with him because he sure as hell wasn't staying in  the damn ground, haunting that pathetic cemetary for all eternity. The  triumph of realizing his sheer determination affected the world around  him enough to still draw his bow and hunt down those who buried him in  their fear, and the elation of knowing his purpose did not end there and  he could still accomplish whatever goal he was brought back to life for  - whenever he figures out what that is. In the meanwhile, he is one  pissed off ghost dragging along his own coffin.