Watari, also called Watari the Flowing, appears throughout the series as a member of the Galaxy Triumvirate alongside Tsuruhashi the Waking and Hashibuto the Angry. He serves the Guild under Miroku's command. Short-statured with brown hair and black eyes, he wears a ninja-inspired outfit in black, white, and red, plus a yellow headband adorned with cherries. His personality thrives on dramatic schemes and deceptions, frequently declaring things "disappointing" and claiming a charming singing voice. His background includes performing on planet ASAKUSA with his Triumvirate comrades. For the Guild, he relentlessly pursues Rikuto April's space compass to aid Miroku's goals, though these missions consistently end in failure. He engages in multiple battles against Rei, losing every encounter.

A fervent admirer of the idol Magical Star Saki, this admiration motivates actions such as orchestrating a fake kidnapping with the Triumvirate to force her exhausted self to take a day off. Rei defeated him in the resulting battle, but Watari deemed it worthwhile after receiving Magical's gratitude and sharing their secret. Later, he purchased a stolen childhood photo of Magical, leading to false theft accusations that Rei helped resolve. Before the Justice Cup tournament, aiming to improve the Triumvirate's reputation, he acquired the new key card Noisy Raven. His first tournament match pitted him against Magical Star Saki herself, resulting in another loss, after which he reverted to dedicated fandom.

During the final conflict against Nakes, the Triumvirate initially fled. Noticing Miroku withheld punishment, they felt remorse for their prior uselessness and returned to infiltrate Nakes, successfully rescuing Miroku alongside Denebola and Rukinosu. In battle, he specializes in white attribute spirits and burst-based tactics, utilizing spirits like Noisy Raven, Burst Turtle, Sheriff-Eagle, Viking-Leiv, The CharismaHero Mibrock-Braver, and The IceHero Mibrock-Baragan, alongside support cards Burst Barrier, Crystallize Illusion, Death Burst, and Flame Burst. Following the series, he joins Miroku on a galaxy-wide apology tour while simultaneously searching for the treasure Ultimate Battle Spirits Neo.