Demyx

Demyx is Organization XIII's number 9. He describes himself as a lousy fighter.

Weapon
Demyx uses his Sitar and his power over Water to fight with.

Real Name
It is believed to be Emyd as an Emyd is a fresh water Tortoise and Demyx controls water so it makes sense if it was Emyd. His name is not yet confirmed it is just speculation.

Kingdom Hearts 358/2 Days
He is a playable character in Kingdom Hearts 358/2 Days.

He is sent to Olympus Colliseum to fight Heartless with Roxas multiple times and In Kingdom Hearts 2 Demyx is first seen in Olympus Colliseum.