Let me know if there are problems with this one. I made most of this up on the spot.

[hider=Light]
Name: Light Void
Age: 17
Gender: Female
Species: Heartless
Appearance:  A 1.7 meter tall pure white humanoid with short blue hair, a large roundish head with no nose and large black and orange eyes, aquatic looking fin ears, a skinny neck, broad chest, skinny biceps with large forearms and equally large cartoony claw like hands that flow into each other. She wears a long white dress with a blue tanktop and blue edges on the bottom section, with an M cut in the bottom section to show her legs of which end in sharp points where she would normally have feet. Her heartless symbol is under her blue tanktop.

Personality: Light has a strong, carefree attitude about things. While intelligent, Light has a difficult time grasping all but the most serious of situations due to her inherent regeneration abilities. This leads her to do things she finds fun more than things that work towards a certain goal sometimes making herself a troublemaker, though not to the expense of her friends. Light is still a heartless though, and while she is strong enough to resist her more basic urges, she still collects the hearts of the Heartless she slays for herself on occasion, usually without even noticing it.

Capabilities: 
Ultimate Regeneration: Light can regenerate from nearly any injury, and gain a resistance to that type of injury depending on how much she needed to regenerate. This resistance lasts for only a few minutes. (Regeneration speed is akin to Deadpool's)
Shadow Slip: Light can travel between worlds by using the shadows, much like any heartless.
Entropy: Light absorbs energy from anything she touches and has innate resistance to energy based attacks.

Weaknesses: 
Ultimate Regeneration can't build resistances to Keyblade or light based attacks.
Entropy acts like a Midas Touch and is always working much to Light's annoyance. This limits her magical abilities severely.
Due to her regeneration, Light is very foolhardy in battle and will often get injured.

History: Light arrived in Sanctuary with no memory of how she got there or her past alongside a wave of other Heartless. While the Heartless attacked random people and were subsequently slain, Light found it pointless and barbaric to attack innocents for their hearts, especially when there was so much more to those people unlike the mindless heartless and other things to do in the world and so much more to life than just collecting hearts for some unknown purpose. Grabbing a blue shirt, she covered her Heartless symbol and blended in with the myriad of other people in the town, defending a few from the Heartless attack. While appalled at the idea of taking hearts from such interesting people, she had no problem with taking the hearts from the heartless she slain, at least when she could safely get her hands on a couple, although she wasn't aware of herself collecting the hearts.
After the fight, she found a strange key shaped sword in her hand alongside a couple hearts she found in her possession. Soon afterwords both disappeared, confusing her greatly.


The Blade
Name: Void's Judgement
Description: A large figure 8 shadow crystal key that emits an eerie negative light. An orb of what can only be called 'space' exists in the hilt. (Whatever that stary purple space like effect is on some characters is called)
Strength: Light's keyblade delivers very powerful blows when she attacks with it. Energy based defenses take an exceptionally large amount of damage from each strike due to Light's Entropy ability.
Weakness: Light's blade is very poor at blocking attacks.
Other: The blade is unaffected by Light's Entropy. While it is the size of a two handed weapon, Light wields it with one hand with no trouble.
[/hider]