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.
Kingdom Hearts 358/2 Days
Demyx is present when Xion joins the Organization. Axel and Roxas often tease Demyx behind his back whilst eating sea-salt ice cream at Station Plaza after missions, quoting how he doesn't have talent, would be "upset" if he lost his sitar, and many other things. Demyx is the first to mention the annihilation of Organization members at Castle Oblivion. Demyx accompanies Roxas on several missions, including one to 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, and is seen playing his sitar on one of the couches in the Grey Area 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
Demyx appears alongside Xemnas, Saïx, Xigbar, Luxord, and Xaldin at Hollow Bastion, and laughs emptily with the rest of the group at Xemnas's empty humor at Sora's expense. Upon the Organization's return from Hollow Bastion, he suggests to let Sora carry out his adventures and that they will take action at the right time. This causes Xigbar to interpret his plan as voluntary and forces this plan upon Demyx instead, despite his plea that they have the "wrong guy" before being lectured by Xaldin for acting as though he has true emotions.
On Xigbar's suggestion, he is sent to the Olympus Coliseum in order to find Sora and "liberate his true disposition", either to keep Sora on the path Organization XIII wish him to take, or try to awaken Roxas. Demyx contacts Hades for his assistance, but no deal was made and Demyx promptly flees, passing by Sora and company, and leaving them puzzled at his outburst. Later, Demyx steals the Olympus Stone, and encounters Sora, Goofy, and Donald within the Underworld once more. Although Sora doesn't understand the meaning of Demyx's objectives nor why Demyx calls him Roxas, Demyx took the next ordered step, which is to attack Sora. However, he is unsuccessful in defeating him and escapes, leaving behind the Olympus Stone. Before Demyx takes his leave, though, he begs "Roxas" to come back to the Organization, leaving Sora even more confused. He 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.
As a Nobody, Demyx lacks a heart, and is therefore incapable of feeling emotions. In spite of this, Demyx behaves as if does possess a heart, an act which Xaldin criticizes him for. As a result, Demyx has a very casual and lazy disposition. He is also cowardly, running from many encounters, including ones with Hades and Sora; and in Kingdom Hearts 358/2 Days, he claims that he finds Larxene quite frightening and also refuses to go searching for a giant Heartless alone.
Due to his laid-back personality, Demyx is on good terms with both Axel and Xigbar, calling Xigbar "Xiggy", and often converses with 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 harbors a strong dislike for 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 dislikes Saïx, calling him "X-face" in one of his Secret Reports, saying that Saïx constantly barks at him to get to work. However, he exhibits 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 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, preferring to stay at The Castle That Never Was or at least be given a reconnaissance mission.
When he is on missions that requires him to engage opponents, Demyx has shown to go about them in a carefree manner. He takes his task of pursuing Sora lightly, reading his orders aloud off of cue cards and complaining about his distaste for fighting in front of the latter. 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. Before his final battle against Sora, however, his true nature as an emotionless entity is revealed, proving that his easygoing, friendly attitude is but a deceptive façade. When he is defeated, however, he screams in agony and defeat, holding his head, and whimpers as he fades back into darkness.
Demyx is portrayed as being weak, cowardly, and lazy, but proves to be no pushover in the battle with Sora. He can be a formidable foe due to his ability to summon water clones that must be defeated within a time limit. 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. Demyx 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 is most often used to play his music instead. 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, which causes him to play his sitar to summon water pillars and, in his Final Limit, release exploding balls of water.
Demyx uses an enormous Sitar, named Arpeggio, a stringed Indian instrument similar to a lute or guitar, in battle (a reference to his fondness for music). 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.