Mari Asai is the protagonist of After Dark. Mari is a 19-year-old college student who studies Chinese. She wears a parka under an oversized letterman jacket and a blue Boston Red Sox baseball cap, which makes her look boyish. Mari grew up in the shadow of her exceptionally beautiful sister, Eri, which caused her to develop an inferiority complex. People at school treated Mari poorly in contrast to Eri, so Mari developed a phobia of public school and dropped out for a time. Subverting her parents’ expectations, she later joined a school primarily for Chinese-speaking children, where she felt more comfortable due to the comparatively lax educational standards. She is going to study abroad in Beijing for at least six months, though she admits she is scared to go. Though she is not fluent, her Chinese language skills allow her to help Guo, the sex worker who was severely beaten by her patron, Shirakawa.

Mari is one of several characters who straddles the line between “light” and “dark.” She is a visitor to the world of nighttime Tokyo; she sometimes stays out all night because she is uncomfortable with Eri’s decision to sleep indefinitely. Mari cannot handle being in the house with her sleeping sister, or with her parents, who pretend that everything is fine.