Demyx is member #9 of Organization XIII and commands the Dancer Nobodies, controls the element of water and wields a Sitar as a weapon. Unlike all other members of Organization XIII, Demyx has a very casual, even friendly demeanor. Despite the great power that goes along with being a member of Organization XIII (not to mention the power needed to attain such a rank), in the brief time he is seen by the player, he runs away from many encounters, including ones from Sora and Hades.
Demyx is notably less serious than other Organization members (or at least appears to be) and seems to take his task of pursuing Sora quite lightly. He doesn't hesitate to read his orders aloud of a cue card in front of Sora, and even complains about having to fight Sora (to Sora's face). This personality is seemingly an act, as in their final encounter Demyx, before battling Sora says to him "Silence traitor" with in a very serious tone, with an equally serious look on his face (whether this is his true nature or if he was just irritated by Sora is unknown). Tetsuya Nomura has confirmed that Demyx will be a playable character in the upcoming DS title "Kingdom Hearts 358/2 Days", although his impact on the story will likely not be any more significant than his impact on Kingdom Hearts II.
Demyx as he confronts Sora, Donald and Goofy at Hollow Bastion
Demyx's role in Kingdom Hearts II is relatively small compared to some of the other Organization members. He is sent after Sora by Xigbar to find him and keep Sora on the path the Organization wants him to take (unknown to Sora, Organization XIII were using him to collect hearts). Demyx also contacts Hades, but it is made clear that no deal was mad, as the player see's Demyx fleeing from the scene, telling himself "Run, run away!", passing by Sora, Donald and Goofy confused at his actions. Demyx later steals the Olympus Stone and soon encounters Sora within the Underworld once more. When Sora confronts Demyx and fails to respond when Demyx calls him "Roxas", Demyx then pulls a card out and reads aloud "If the subject fails to respond, use aggression to liberate his true disposition" and then remarks "Did they ever choose the wrong guy for this one". After a brief battle with Demyx and his water bodies, Demyx retreats instead of fighting on.
Demyx later confronts Sora in Hollow Bastion during the heartless invasion and again seems laidback and even friendly even in his confrontation. Sora remains hostile however and claims that Demyx is unable to fight. Despite the insult, Demyx remains friendly, but when Sora claims that Nobodies have no hear to which Demyx replies "Ooh we do too have hearts, don't be mad!". Donald then says that he can't trick them and Demyx suddenly becomes dead serious and quiets Sora and co. by saying "Silence traitor" is a significantly more serious, darker tone. He then attacks Sora, this time he fights much more seriously, but is defeated again, this time fatally. He then fades into darkness and then, interestingly, also unlike other organizations is engulfed in blue light and then a column of water before vanishing.