Demyx, the Melodious Nocturne (夜想のしらべ Yasō no Shirabe , lit. Nocturnal Melody), is Rank IX within the original 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.
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.
 Between Kingdom Hearts Birth by Sleep and Kingdom Hearts 358/2 Days
On an unknown world, Demyx's original persona became a Heartless, his strong will allowing his body to continue on as his Nobody, Demyx. He was later found by an unknown member and became the ninth member of Organization XIII.
At a later time in his room, Demyx was fixing his hair in front of the mirror, compelled by something he had done when he was human. When he was finished, Demyx played around on his sitar, but he was interrupted by Xaldin, who told him to keep it down because he wanted to sleep.
Annoyed, Xaldin questioned why Demyx clung to his human memories, but received no reply. Xaldin told him that Nobodies needed to sleep because a terrible force was moving that 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 was 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. Demyx accompanies 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. When Roxas approaches Demyx, he willingly admits that he "isn't cut out for combat" and would rather perform recon. He is shocked to learn of Xion's true nature when Xemnas describes the Replica Program to the Organization.
 Kingdom Hearts II
Later, he is sent after Sora on suggestion by Xigbar to find him and "liberate his true disposition," either to keep Sora on the path Organization XIII wish him to take, or try to awaken Roxas. 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. Before he takes his leave though, he begs "Roxas" to come back to the Organization, leaving Sora even more confused.
Demyx briefly returns to explain the nature of the Absent Silhouettes, remarking that by defeating the corresponding enemies, Roxas will "awaken" inside Sora.
Later, during the Battle of the 1000 Heartless at Hollow Bastion, Demyx returns to obstruct Sora, despite greeting him in a rather friendly manner. Although Sora and his friends respond by dismissing Demyx as a threat, the Nobody warns them not to judge by appearances, and his carefree air vanishes completely when Sora remarks that Nobodies lack hearts. Although he attacks the "traitor" with all his strength, he is defeated in the end and fatally wounded. As he laments his loss, he fades into darkness.
Demyx wears a black cloak similar in style to Xigbar's, with somewhat closer-fitting sleeves and pointed shoulder pads, though Demyx's shoulder pads are more noticeable than Xigbar's. His black gloves and boots are normal. He has green eyes and dirty-blond hair, the latter of which is quite distinctive. Demyx's hair has an odd mullet-like style, and he has several individual bangs that fall over his face. He is rather youthful in appearance when compared to most of the other members.
Demyx, unlike the other Organization members, has a very casual, cowardly, and lazy personality—though this is simply an act since Nobodies are free of all emotion and thus cannot feel fear. He runs from many encounters, including ones with Hades and Sora; and in Kingdom Hearts 358/2 Days, he states that he finds Larxene quite frightening and also refuses to go searching for a giant Heartless alone. Along with Xigbar, Axel, and Larxene, Demyx is portrayed as one of the more comical members of the Organization, although unlike the other three who have that role due to their sarcasm and wisecracks, Demyx has this role because of his laziness.
With his laid-back personality and despite his often sitting around, Demyx seems to be on good terms with both Axel and Xigbar, calling Xigbar "Xiggy", and is often shown talking to him in the Grey Area. Although Axel and Roxas talk about him behind his back, Axel defends him after Roxas comments that Demyx doesn't do anything, stating that Demyx specializes in recon.
Demyx seems to dislike working as much as he dislikes fighting; he even goes as far as manipulating Roxas, dumping his missions on the younger Nobody and going off to have fun. He complains about how he "isn't cut out for combat" and even considers sweat gross. Often, he wishes that Xemnas wouldn't make him fight and says that he'd prefer to stay at The Castle That Never Was or at least be given a recon mission.
Demyx seems to strongly dislike Larxene, calling her a witch. The feeling is apparently mutual as she has referred to him as lazy and useless. When news gets back from Castle Oblivion, he seems to be glad that she has died, showing that even he can be a bit cruel and crass without a heart. He also seems to dislike Saïx, calling him "X-face" in one of his Secret Reports, saying that Saïx constantly barks at him to get to work. However, he does show some care toward Xion compared to the other members who have no care for her at all, showing that he may be a bit kinder than them, although he thought it was "intense" when he found out that Xion was a "puppet".
Demyx is much less serious and more carefree than the other Organization members, as well as selfish, and seems to take his task of pursuing Sora lightly. He doesn't hesitate to read his orders aloud off of cue cards in front of Sora nor does he hesitate to complain about his distaste for fighting. His cries in battle after being hit are also more characteristic of "yelps" than those of the other members, and are exclamations rather than reactions. In his final battle against Sora, however, his true nature as an emotionless entity is revealed, proving that his light attitude is a farce. Yet when he is defeated, he screams in agony and defeat, holding his head, and whimpers as he fades back into darkness. This may indicate that he did not fully reject his fake emotions, but, overall, he seems to have come to terms with his lack of emotions.
Demyx is portrayed as being weak, cowardly, and lazy, but proves to be no pushover in the battle with Sora. Most players have trouble defeating him due to his ability to summon water clones that come with a time limit to defeat all of them. Demyx, being a musician, uses music to control the water around him and to summon "water forms", objects shaped out of water into musical notes or copies of himself, which move about the battle area to hit Sora, forcing him to defeat the forms in a certain amount of time to avoid an automatic loss. He can also summon violent geysers to erupt from the ground, forming strong streams of water which damage Sora and throw him into the air. His sitar is rarely used as a weapon, but most often used to play his music instead. However, Demyx does use one move using his sitar as a weapon, but it does little damage to Sora. Demyx does little fighting by himself, instead relying on his control of water to make it fight for him.
In Kingdom Hearts 358/2 Days, Demyx has a Limit Break called Wave Gigs. During Wave Gigs, the player must press and time the attack button in specific "beats" as Demyx jams out on his sitar, summoning up large pillars of water to rise and strike enemies. The more efficiently the button inputs are performed, the faster and more plentiful the attack will be. Enemies that are hit by the pillars take water damage. Meanwhile, Demyx's movement speed is reduced to a slow march, allowing him to slowly advance on an enemy as he attacks, but not much else. His falling is also slowed.
During Demyx's Final Limit, Demyx releases a large ball of water while jamming out on his sitar. The ball creates a watery explosion that knocks enemies back and deals water damage.
Demyx is very open about his dislike of fighting, and prefers music instead. So, he carries an enormous Sitar, named Arpeggio, a stringed Indian instrument similar to a lute or guitar, into battle. The Sitar itself is shaped like the Nobody insignia, in varying shades of blue.
With it, he can cause water to take the form of people and musical notes, making them "dance" to his music. Although not very useful in battle itself, it can cause some damage if swung at an enemy. He mostly uses its strings to control water and make it fight for him.