Demyx controls water, and uses his weapon, the Sitar, as a musical instrument to make water fight for him in battle. He commands the Dancer Nobodies.
Demyx plays a minor role in Kingdom Hearts II, where he is tasked with tracking down Sora to liberate Roxas's "true disposition". Although he dislikes fighting, he is directed to use "aggression" against Sora and his companions to bring out Roxas. Demyx is a playable character in Mission Mode in Kingdom Hearts 358/2 Days. [Source: wikia]
You can download the paper craft toy here: Kingdom Hearts - Chibi Demyx Free Paper Toy Download