Xemnas is the founding member or Organization XIII, and number One. He came up with the plan to create hearts using Kingdom Hearts to become whole again. He uses two ethereal red blades (which look just like Lighsabers, so I think he might be a Sith... dunno.) and the element of nothingnes.
Now, I know that there's the whole Ansem-Xemnas-Riku-Diz-Xehanort-Terra paradox and stuff, everyone looks the same at some point, and everyone is called Ansem at some point... It's really confusing, and I plan to explain it sometime, but just to clarify, I'm just gonna talk about Xemnas, the leader of Organization XIII, kay?
He is evil, rarely appears in 358/2 Days, letting Saix transmit his orders in his stead, and is the final boss in KH II. You'll fight him after you defeat everyone else in the Land That never Was, in the highest spot in the building. He has five stages in total, and you fight him with Sora and Riku. The first is easy, it takes place in the same spot where Riku and Roxas fought, and he is weak, do not have strong attacks. the second stage is the demon king, he is in a throne. use drives and win easily. The third is like a gummi ship travel, just recharge and fire at him. there are close to no enemies, and he will have become a giant flying thing that does not attack you. On the fourth, he's in the throne again, but this time he has some lighting attacks, and can throw you into space. if he does so, use reaction commands to land in buildings and come back aboard. be careful with the lighting that keeps you from hitting him.
The fifth and final stage is in a black and white room.Xemnas will have changed clothes. the floor moves in a circular confusing motion, and his attacks are strong. This stage is hard. use reaction command "reversal" to get close to him, and try to avoid his clone he'll create, focus on himself. When he catches Sora and you control Riku, use dark aura to block his attacks and get to him to free Sora with the reaction command. When he starts shooting a freaking lot of lasers, hit triangle and x at the same time repeteadly to reflect them. you have to hit him with a combo to win, killing him.
Once you did, congratulations, you finished KH II!
He's a very strong oponent, and really hard to beat. In KH 3D (Dream Drop Distance) he appears in a younger but already Nobody version.
See what I said? HE USES TWO LIGHTSABERS! HE'S A SITH LORD!