Demyx

Majority of information taken from Kingdom Hearts Wiki-Demyx.

Demyx the Melodious Nocturne (夜想のしらべ, Yasō no Shirabe, lit. Nocturnal Poem), is Rank IX within Organization XIII. He 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.

Kingdom Hearts Novels
In his own room at an unknown time, Demyx was fixing his hair in front of the mirror, something he did from when he was a human made him do it. Finishing setting his hair, Demyx picked up the sitar and strummed it. He plays and plays, but he was interrupted by Xaldin, who told him to keep it down because he wanted to sleep.

Annoyed, Xaldin questioned why Demyx clings to his human memories, but he didn't reply. Xaldin told him that Nobodies need to sleep because a terrible force is moving this world and they need rest to store up power. Demyx did not reply once again as Xaldin left the room, displeased.

Kingdom Hearts 358/2 Days
Demyx plays a slightly larger role in Kingdom Hearts 358/2 Days, present when Xion joined the Organization. Often teased behind his back by Axel and Roxas whilst they eat sea-salt ice cream at the Station Plaza after missions, they quote how he doesn't have talent, would be "upset" if he lost his sitar, along with many other things. Demyx is the first to mention the annihilation of Organization members that took place at Castle Oblivion, accompany Roxas on several missions including one to the Olympus Coliseum where they take down a Flare Note, and one to Halloween Town where they tackle a Dual Blade. Demyx enjoys picking on Xion with Xigbar, seen playing his sitar on one of the couches in the Grey Room before many of Roxas's missions. In the manga adaption, in one of the early chapters, he is also seen playing a prank on Axel, where he puts a farting pillow on Axel's seat. When Roxas approaches Demyx, he willingly admits that he "isn't cut out for combat" and would rather perform recon.

Kingdom Hearts II
Demyx appears in Hollow Bastion with all of the other Organization members, but he does not contact Sora. However, you can hear Demyx's higher-pitched, nervous laugh in the background.

Demyx's role in the Organization's plans is minor. He is sent after Sora on suggestion by Xigbar to find him and "liberate his true disposition", that is, to keep Sora on the path Organization XIII wish him to take. He also contacts Hades, but no deal was made and Demyx promptly flees, saying "Run, run away!", passing by Sora and co. and leaving them puzzled at his outburst. Later, Demyx steals the Olympus Stone, and soon encounters Sora within the Underworld once more. Although Sora doesn't understand the meaning of Demyx's objectives, nor why Demyx calls him Roxas, Demyx is unable to fulfill them and took the next ordered step, which is to attack Sora. Demyx at first only summons "forms" of water - water shaped into himself and musical notes, which then attacked Sora, Goofy and Donald - which he controlled with his sitar. He is unsuccessful in defeating Sora with his water forms, however, and escapes instead of continuing the fight.

Demyx confronts Sora later in Hollow Bastion during the Battle of the 1000 Heartless, again greeting Sora in a rather friendly manner. However, Sora and his companions' response is hostile, even claiming that Demyx is unable to fight. Demyx still maintains his personality despite the insult, with only a "You shouldn't judge anyone by appearance" in a slightly deeper tone. But when Sora and co. claim that the Nobodies do not have hearts, Demyx's normal carefree air vanishes, and he silences them with a much darker tone and attacks the "traitor" again with all his strength. In the end, however, Demyx is defeated again and fatally wounded. After lamenting his loss, he fades into darkness, the first Organization XIII member to do so in Kingdom Hearts II.

Day 97: Sooo Bored
Author: Demyx

''Every day it's work, work, work. I'm dead bored, and I'd kill for some vacay. A Nobody needs his rest, am I right? And when I get it, it's gonna be nothing but me and my sitar. Sigh...''

Day 153: This Is Gonna Be a Hit
Author: Demyx

''I've got me a new hit single! X-face is always barking at me to work, but what's he expect me to do against a monster like that? I mean, stop and think about skill sets when you divide up the labor already!''