Trickmaster

The Trickmaster is a boss from Kingdom Hearts and Kingdom Hearts: Chain of Memories.

Kingdom Hearts
The Trickmaster is a special Heartless with the ability to steal people's memories. It was sent to find the Wonderland keyhole and destroy Wonderland's heart. It was also sent to capture Alice. Though it succeeded in the latter, the Trickmaster was defeated before it could destroy Wonderland. Throughout most of the adventure in Wonderland, the Trickmaster is referred to as the shadows or as the darkness (especially by the Cheshire Cat). It was the one who attempted to steal the heart of the Queen of Hearts.

Kingdom Hearts: Final Mix
In Kingdom Hearts: Final Mix, Trickmaster was recolored with pink replacing red and light purple in place of black, giving it a more Wonderland-ish look.

Kingdom Hearts: Chain of Memories
Trickmaster briefly appears as a boss and a thief, stealing the Queen's memory.

Trivia

 * Despite appearing to be one of the largest Heartless Sora fought, in reality it was roughly the size of Donald Duck; Sora battled it while they were small.