Matsuri Shiina is the older sister of Aki Shiina, sharing his distinctive green eyes. She possesses pale skin and long grey hair reaching her knees. Standing 140cm tall, she typically wears a white-black dress and cross-shaped hair accessories.

Her personality is energetic, perverted, possessive, and intensely overprotective, especially concerning Aki, revealing a significant brother complex. She exhibits obsessive affection and jealousy when other girls interact with him, sometimes manifesting psychopathic tendencies. Matsuri frequently forces Aki to cross-dress, referring to him as her "sister" and treating him as female regardless of his identity. Her overbearing control contributed to Aki's choice to move away.

While responsible, Matsuri retains a childish side, blurring sisterly care with helicopter parenting. She evaluates others' caretaking abilities, such as assessing Ayaka Sunohara's skills, even experiencing ear cleaning firsthand. Despite her extreme methods involving control and manipulation, Matsuri demonstrates concern for Aki's well-being.

Later manga chapters depict Aki growing taller than Matsuri and developing more boyish features. Matsuri's behavior and dynamics with her brother remain consistent throughout official media.