Demyx was first seen investigating the Olympus Colliseum Sora commented he wasn't a good fighter and being a broken record.
During the battle against a huge Heartless army Demyx appeared revealing to us a more opposite nature and he has powerful water abilities(it goes to show that any member of the Organization is powerful/deadly)He the only musician related member because his weapon is called a "sitar" which similar to a guitar but different in tone, method and origin.
He has seven attacks:
Water dance-creates water duplicates
Water Column-creates water columns in one of three patterns
Water Wall-creates a wall of water and swings sitar at close targets
Water Column dance-Creates water pillars while swinging sitar
Water ball-attack-fires exploding balls of water
Rapid water ball-lobs exploding water balls overhead
Rave- runs up to Sora and lash out with sitar
But, despite of all these attacks Sora, Donald and Goofy worked together to beat Demyx and he is the first Organization member in KH2 to fade into Oblivion.