Demyx

"Dance water Dance!"

- Demyx

"You shouldn't judge anyone by appearance."

- Demyx to Sora and his friends

"Silence Traitor!"

- Demyx when he drops his emotional ploy.

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.

Past
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 is presented when Xion joined the Organization. Demyx sometimes accompanies Roxas on several missions. One to them is when they traveled to the Olympus Coliseum where they took down a Flare Note. Another one is when they went to Halloween Town where they tackle a Dual Blade. 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 was sent after Sora on suggestion by Xigbar to find him and to keep Sora on the path that the Organization XIII wish him to take. He also contacts Hades, but no deal was made and Demyx promptly flees, while passing by Sora and his friend. Later on, 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. At first Demyx attacked Sora by only summoning "forms" of water. These were water that took the shape into himself and musical notes, which then attacked Sora, and his friends. These water creatures are controlled with the use of his sitar. Sora managed to defeat Demyx's water creatures, however,  Demyx escapes instead of continuing the fight.

Demyx confronts Sora later in Hollow Bastion during the Battle of 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 tried to show some bravery by saying, "You shouldn't judge anyone by appearance," but when Sora and friends took out their weapons, Demyx began to cower, claiming that Organization XIII sent the wrong guy. Sora and his friends then claimed that the Nobodies do not have hearts, Demyx's normal carefree attitude 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.

Trivia

 * Demyx's name is never spoken in the original Kingdom Hearts II, so fans were divided about how to pronounce his name (some including DEH-mix, DEE-mix, and Deh-MEEKS). His name was pronounced in Japanese in Kingdom Hearts II FInal Mix, but his name's official English pronunciation was not clarified until HD 1.5 ReMIX, which pronunciates it as DEH-mix.
 * Prior to HD 1.5 ReMIX, Demyx was one of three members of Organization XIII whose name wasn't spoken out loud, the other two being Xigbar (prior to Dream Drop Distance) and Luxord.
 * Coincidentally, Airwalk produces a shoe model named Demyx (one likely focused towards women with its design), and another named Roxas.
 * Despite wielding water, Demyx has a weakness against Fire magic. This is because in the magic properties of Kingdom Hearts II, water is classified as form of Blizzard magic. However, in Kingdom Hearts 358/2 Days, water and blizzard are two different types of attacks and magic.
 * Most of Demyx's weapons are named after musical or rock and roll terms.