Demyx (デミックス, Demikkusu?) is the Organization's reconnaissance who is also referred to as the "Melodious Nocturne" (夜想のしらべ, Yasō no Shirabe?), a slacker who would rather play his sitar "Arpeggio" (アルペジオ, Arupejio?) than go on missions, especially those that require physical labor. Though he admits he's not much of a fighter, Demyx has power over water, using it to create duplicates of himself. He also has control over the Dancer Nobodies.[7] Demyx is voiced by Kenichi Suzumura and Ryan O'Donohue in Japanese and English respectively.[11][13]

In his first chronological appearance in 358/2 Days, Demyx reluctantly participates in the missions Saix assigns to him out of fear of what Xemnas could do to him.[46] He then appeared in Kingdom Hearts II at Hollow Bastion with the other Organization XIII members. Soon after, much to his chagrin, Demyx was assigned to look over Sora's progress in the Organization's plan, and confronted Sora in The Underworld while in possession of Hercules' medallion. After attempting to talk to Roxas through Sora, Demyx sends his water copies after him and leaves after losing. Demyx later crosses paths with the protagonists again at Hollow Bastion, and battles Sora and his friends after they enrage him enough about his nature as a Nobody.[7] Demyx, however, is defeated and fades into darkness.