Yuni is a black plush kitten resembling characters such as Mew, Suu, and Peko, marked by a white muzzle and a red star on one ear. He typically wears a black cloak fastened with a red ribbon and a black hat featuring goggles. His speech pattern includes the verbal tic "~nyui".

Originating from the sky as an unused plush, Yuni fell to the human world and endured prolonged isolation after remaining undiscovered. The Queen of Nightmares granted him movement and speech, securing his deep loyalty. During his solitude, he crafted two cherished plush companions, Tsugi and Hagi.

Initially serving as an antagonist under the Queen, Yuni employs Black Abyss stones to corrupt dreams into nightmares by spreading "Black Schemas". With Tsugi and Hagi's aid, he transports individuals between dreams. In *Mewkledreamy Mix!*, he accidentally animates another plush—Pachipachi-Bu—by inserting a "Yunicchi Abyss" while attempting to replace a character. Yuni values Pachipachi-Bu as deeply as Tsugi and Hagi.

Yuni's partnerships shift across the series. He first dwells in a makeshift shelter beneath a bridge, later relocating to Haruhito Sugiyama's home under the Queen's orders. By Episode 24 of the first season, they formalize their partnership, though Yuni departs on a journey afterward. He returns in *Mewkledreamy Mix!*, transferring his partnership to Haruhito's brother, Akihito Sugiyama. His allegiance also shifts from the Queen to a new superior, Akumu, in *Mix!*.

Despite mischievous tendencies, Yuni displays genuine care for his partners and created companions. His actions occasionally reveal clumsiness, contrasting his antagonistic role.