My only comments:
Does heartless actually have bellybuttons?

[EDIT]
I changed his power

Name: Sin
Species: Joker
Type: Dark Knight
Homeworld: Wonderlands (Despites having lost his heart in Radiant Garden)

Weapons: Mourneblade.

No matter how holy or ice elemental this sword may look, it is actually considered evil. Of course, no heartless could be born with such a weapon, which makes the others wonders where he got it. This blade actually drains the souls as it cut and slices, directly making Sin absorb the energy.
He fights mostly by making his sword very very unattached to gravity, making her very quick and light, then striking with her or throwing her and making her extremely heavy at the last moment, giving out a powerful blow.

Ability/Power: Gravity

The dark term of Gravity (NOT DARKNESS), with this, he can hold someone on a wall, on the ceiling, on the floor or simply making them light and then hit them to fly away.
This is kinda like telekinesis, but shares many different, for instance: he cannot move things, he can only make them light or heavy to a point where they fly away or are rocked to the ground.

Strength/Weakness

Magic, Speed/Power, Defense

Personality

Sin is more than bipolar, yet does not have two-side personality. He only have one mind, with two face, as he is always conscious of what he's doing.
His usual is a laid-back, joking jester, funny leader, while the other side is a cruel, sadistic and heartless monster.

He does not believe in Kingdom Hearts, he only believe that if everyone falls in darkness, then there will be no more pain.

Facts

-He never sleep.

-He almost always have a permanent vicious/cruel grin.

-He sit in some weird position, some even defying laws of gravity (He DO comes from wonderland).

-The bells on his outfit aren't made of metal, but instead of a material that shine slightly. They do not makes sound, but tingle like normals bells. (Some even said they could hear little scream, saying that fairies are trapped inside the bells, which would explain why he can't stand people touching them.).

-Most of all: He is unpredictable. It is impossible to tell what he'll do the minute after.