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Yes. After completing Sora's story Riku's mode will be available. It's how Riku got to know things before KHII arrived and got an upper hand.

The two Nobodies on his side are Zexion and Lexaeus. Zex being the short blue haired pouty one and Lex being the silent hero. (Quite literally; doesn't really talk much.) Zex is known for getting others to take care of things for him instead of dirtying his hands himself. Often misunderstood as being emo by the fans. Lex is also misunderstood as being a 'dumb giant that speaks like a kindergardener'.

Vexen also appears. It was how he gathered the data to create a Riku replica the real Riku meets and is parading around in Sora's story claiming to be the real Riku. (Due to Namine's influence.)

That's summed up. If you want the whole story just ask or look around.

Marluxia isn't hard! crying Pity he isn't around anymore.

Zexion's weapon is unconfirmed. Tetsuya wanted an air of mystery around it therefore why he smashed his tombstone in the Proof of Existence. The closest his strength comes to is being able to sense Darkness (How he let the others know Riku was in the castle), and manipulating others through illusion.

Riku never appears in Sora's story. That was the Riku Replica all the time. Riku eliminated him in his own story after Axel brought the Replica over to Zexion. He said that if the Replica absorbs Zexion's power he's grow as strong as the real Riku and be whole. That's how Zexion was eilimated; the screen fades out and you hear a slashing noise. (This is way later in his story.)

Marluxia wanted to overthrow the Org using Sora. He gathered whoever was willing to work with him to Castle Oblivion to get things started. Zexion, Lexaeus, and Vexen, however (who worked on both sides) were plotting against him on his own plan, instead going after Riku to counteract Marluxia once he got control of Sora. Though, once Sora defeated Marluxia, Riku wasn't needed anymore. Lexaeus went out to destroy him for having no use for him anymore. Riku won the battle but was thrown into the realm of Darkness due to the Silent Hero's last attack. Mickey came and helped him out. (He comes time to time...)

After Zexion was left all by himself he was forced into action. Rather than fight Riku one on one with strength as Lexaeus did he disguised himself as Sora at Destiny Islands to manipulate him into believing he was a backstabbing traitor. Riku saw through it, though. He had acquired the ability to recognize Darkness through his senses through Xehanort. Zexion, injured, had fled only to be thwarted by Axel and the Riku Replica not to long after.

Good luck beating that. I was having trouble beating Anmas at the end. Really piss me off.

wait i didnt beat the game its no fair....i wanted to beat it...

I think that Zexion's still alive. Squenix has a weird way of throwing in cherries like that all the time, so I think that he'll be back in KHIII. I mean, you know absolutely nothing about him--he has no power or weapon. All you know is that he can make himself look like anyone he wants to (he turned into Sora for a while in Destiny Islands) and that he tried to trap Riku in the light...yeah, I think that he'll come back.