Kū Mitsuhashi, son of former L.M.O. DJ unit member Shō Mitsuhashi, spent years hunting for his father after Shō left the unit, moved overseas, and caused L.M.O.'s disbandment through lost contact. Following Shō's contract termination, Kū became legal guardian for the members of Abyssmare during their time in Japan. Shō later resurfaced as an antagonist, forming Abyssmare and treating its members as tools for his ambitions. After Shō's defeat and subsequent departure from Japan, Kū officially assumed all rights to Abyssmare. Despite holding these rights, he granted the four members full operational control of the unit; his current involvement remains unspecified.

Kū is acknowledged as the founder and host of the in-universe D4 FES events. He possesses short platinum blonde hair and blue eyes, typically depicted in professional attire. His surname "Mitsuhashi" (三橋) combines "three" and "bridge," while his given name "Kū" (空) means "sky" or implies vast emptiness.