Akina Kubo, Nagisa Kubo's older sister, stands 167 cm tall with short red hair and red eyes, possessing a notably taller and more developed physique than Nagisa. Born on December 21, she works at a bookstore somewhat distant from home and keeps a relaxed attitude about punctuality in her daily life.

Playful and outgoing, Akina frequently teases both Nagisa and Junta Shiraishi, pushing boundaries with bold remarks or sudden actions like unexpected hugs and cheeky comments about relationships. This tendency sometimes escalates after drinking alcohol at home, leading to behaviors such as walking around in minimal clothing that embarrasses Nagisa. Despite her teasing nature, Akina is a deeply caring and reliable older sister, offering emotional support during Nagisa's failures or insecurities, like comforting her after a childhood incident with broken eggs or encouraging her through cooking mishaps.

Akina exhibits exceptional competence in most tasks she attempts, partly driven by a desire to meet expectations as the older sibling. She achieved top academic results in school and excels at cooking, contrasting with Nagisa's struggles. Like her family, she can easily perceive Junta despite his usual invisibility to others, using this ability to facilitate interactions—returning lost items through Nagisa or observing his relationship with her sister.

She shares a close bond with her cousin Saki Kubo, spending significant time together and often eliciting exasperated reactions from Saki with audacious comments. Akina subtly supports Nagisa and Junta's developing relationship, offering indirect encouragement while respecting their pace. She holds a fondness for gratin, hides her exact age though confirmed to be in her twenties, and experiences bittersweet pride when noticing Nagisa's increasing independence, such as trying coffee for the first time during a date.