Demyx is a pretty awesome character too, he's the biggest coward in the organization, is always complaining about the missions he gets sent to, and that he does not like to use violence. Because of his childish way, he's constantly bullied by Larxene.
He uses a Sitar, and the element of water, creating water forms. He's also funny, and always when he created the forms he says "dance, water, dance". Because of his indisposition for violence, his mission is to follow Sora, and check his progress in destroying heartless. In 358/2 Days, he is always sitting in a sofa and complaining about having to fight.
You'll fight him in the Underworld in Olimpus Coliseum and in Hollow Bastion, right before the battle of a thousand heartless. The first time is easy, but the second one is hard. His attacks are basically grat pillars of water, so you just dodge, use guard or reflect when he shoots water balls, use reaction commands and try to get close enough to hit him. The tricky part is the part when he creates water forms and you have a impossibly short time to destroy a hundred of them. If you manage to accomplish this, then you'll be set for the rest of the battle.