Youhei Mashiba is the 26-year-old twin brother of Mitsuki Mashiba, who is the younger twin. He intentionally remains unemployed to stay available for Kyouko Shirafuji, stemming from his past membership in her delinquent group. This connection leads him to occasionally take temporary shifts at the Wagnaria family restaurant during staffing shortages. On the job, he overflows with enthusiasm for assigned tasks but proves clumsy and ineffective, struggling with basic responsibilities like memorizing orders.

Physically, Mashiba sports gray hair tied in a ponytail, sienna brown eyes, and a visible snaggletooth. His typical attire consists of a green jumpsuit over a white t-shirt paired with brown shoes. His speech and demeanor are hyperactive and informal, heavily laced with slang.

His relationship with his twin sister is contentious, marked by mutual hostility and frequent arguments; he perceives her as arrogant and disrespectful, especially given her younger age. Conversely, a significant dynamic develops with Kozue Takanashi after he casually remarks on her appearance post-breakup. Unlike her past rejections, his refusal to reciprocate her romantic interest is based solely on his unemployment and inability to provide financially. This distinction prompts Kozue to persistently propose marriage to him. Their relationship evolves, with implications of cohabitation or physical intimacy by the series' conclusion.