Kadaj

Kadaj is the antagonist of Final Fantasy VII: Advent Children. He is vaguely childish and violent. In Distance: The Making of Advent Children, one of the special features on the DVD, Tetsuya Nomura states that Kadaj personifies the insecurity of a young man.

WARNING: PLOT/ENDING SPOILERS AHEAD!

Kadaj is the leader of the trio of villains featured in the movie, the other members are Loz and Yazoo. The three are refered to as "remnants" of Sephiroth, and are searching for Jenova so they "can be whole again." It isn't revealed what he means until the end, when Kadaj fuses with Jenova's head and becomes Sephiroth.

SPOILERS END HERE

Kadaj seems jealous of Sephiroth. At one point he says that it's unbearable to think that Jenova would want Sephiroth more than him.