Ryū Natsume is an eccentric and energetic apprentice bartender at BAR 4/7, claiming an unverified age of approximately 800 years. Under Yohei Kanbayashi's guidance, he concocts bizarre drinks through unconventional combinations. His distinctive appearance features asymmetrical white hair, multiple piercings (including eyebrow and tongue), and extensive tattoos. He favors cat-themed clothing like suspenders with cat paw brooches and cat-adorned loafers. His background is obscured by fragmented memories. Fragments recall "a big home with white walls" with many people, later revealed as the Alter Trigger facility where he underwent phantometal experimentation as subject "Zero." Fellow subjects Itsuki and Rokuta recognize this designation, though Ryū himself recalls neither the name nor his time there. Approximately nine years before the main events, he escaped the facility and encountered Yohei near a hospital while searching for an older brother figure who showed him kindness during experimentation. A brief reunion occurred before Ryū was returned to the facility. Naoakira Saimon discovered Ryū later, bringing him into BAR 4/7's care before taking in Shiki Ando. Ryū addresses Naoakira as "Boss" and Yohei as "Master," reflecting their caretaker roles. His personality blends childish antics—nonsensical speech, biting Shiki as a morning ritual, creating unsettling drinks—with flashes of unexpected perceptiveness. He demonstrates knowledge of private conversations he didn't witness, locates individuals via unexplained "electromagnetic waves," and offers cryptic warnings about futures, like predicting Hajun's impending "metal erosion." He typically deflects questions about these insights with indirect or absurd responses. A significant anomaly is his immunity to "trap reactions," the traumatic illusions induced by phantometal overuse. This results from the Alter Trigger experiments and causes him emotional distress, fostering feelings of isolation and a fear that it signifies an inability to form genuine connections. Close friend Hokusai Masaki ("Fuu-chan") describes him as "lonely like a stray cat," existing in a paradoxical state of being "connected with everyone, but also can’t connect with anyone." His daily schedule reflects a chaotic routine, mixing ordinary activities with idiosyncratic elements like "charismatic bartender time," nighttime strolls, and solo singing. He shows affection for cats, evidenced by rescuing a stray kitten and integrating it into BAR 4/7 when no owner was found. His phantometal accessory, the Compliance Tongue Piercing, activates when licked. Ryū fabricates absurd milestones in his self-reported timeline, such as "winning Pluto’s Spinach Eating Championship" at age 623 and "graduating from humanity" at 189. These contrast with underlying trauma hinted by hashtags like #SomethingGotLeftBehind, #ABigHouseWithTallWhiteWalls, and #MemoriesAllMixedUpLikeJuiceCocktail. Despite his outward exuberance and motto—"Whatever is fun is fine by me!"—these elements point to unresolved pain from his past.