Axel seems to have taken a liking to killing the other awesome guys, because after he kills Vexen, he also Kills Zexion, another genius character. I specially like Zexion because he's the kind of quiet smart character, who makes the schemes, like his title suggests.
He's killed because he knew a bit too much about Marluxia's group's plan to take over the Organization, and so Axel was ordered to eliminate him as well, using a replica of Riku to strangle him to death. he also appears in CoM Re: Reverse Rebirth, where he turns into Sora to try and confuse Riku (he has the ellement of illusion), but he is defeated.
Zexion uses a book to fight, other thing I find really awesome and badass. he's also enigmate, with his closed expression and hair... He also used to work in Radian Gardens with Ansem. His human name was Ienzo.