Masaki Kobayashi owns and operates the French-named café restaurant L'Amant. He insists on being called Marie or Mari-san, rejecting his given name. Long curly black hair, usually tied in a ponytail, and a light trimmed mustache define his appearance. He maintains a hairless, well-moisturized body and wears expensive cologne. His typical work attire is a waiter's button shirt and bowtie with very short trousers and leggings, though he sometimes wears drag, a fundoshi, or a leather thong. Extensive Yakuza-style tattoos cover his back and parts of his chest, limiting him to bathing at a specific local bathhouse run by a regular customer. Rumors among bathhouse companions about his anatomy earned him the nickname "The Big Gun of Akamori Shopping Street."

Born the eldest son of a Yakuza leader, he held significant gang roles and was heir apparent. During this time, he concealed his homosexuality, leading a double life. A pivotal moment came during negotiations when he met Jin Katsuragi, a lieutenant from the rival Kodan Gang, and experienced love at first sight. Though their relationship ultimately ended, pursuing it catalyzed his decision to leave the Yakuza. This choice angered his father, but he openly embraced his identity as an ordinary feminine man. He later established L'Amant, hiring Fumiya Kurimoto and Ayano.

His personality is flamboyant, cheerful, and effeminate in speech, though he occasionally reverts to a masculine gangster persona. He maintains high self-care with regular exercise, including yoga. Empathetic, he detects emotional distress and offers advice, sometimes requesting payment via drink purchases. He maintains strict confidentiality, even with conflicting viewpoints from patrons like Rui and Hina Tachibana. While prohibiting violence in his café, he finds entertainment in customers' romantic disputes. He openly flirts, frequently asking men if they're single and teasing regulars like Natsuo Fujii. He holds a long-standing rivalry with "Erika" (real name Etsuo Kikuchi) dating back to their time working in gay bars.

Within the narrative, he functions as a trusted confidant for L'Amant's regulars, including Natsuo Fujii, Rui Tachibana, and Hina Tachibana. He counsels Natsuo on various matters, especially relationships, and supports Hina as she confides her love for Natsuo and commitment to his happiness regardless of reciprocity. His guidance stems from his own experiences with complex relationships and personal reinvention.