Yasuko is Ryūji's thirty-three-year-old mother, though she refers to herself as eternally twenty-three; she has large, F-cup-sized breasts. Though she tells Ryūji his father died before he was born, the truth is they were never married; he left her before Ryūji was born, and she raised her son by herself. She is also klutzy and is never entirely in this world, as she is usually half asleep.
She works at a hostess bar named Bishamontengoku where she goes by the name "Mirano" (魅羅乃), and singlehandedly supports her two-member family financially. However, she has a childish personality and since she works, she entrusts the household chores to Ryūji, although she gives him some encouraging advice to help his relationship with Taiga Aisaka. She instantly takes a liking to Taiga and already regards her as a member of her family, getting depressed when Taiga will not appear for dinner. However, she does get angry when woken up too early since she usually comes home very late.
Despite the childish, air-headed behavior, Yasuko proves to have a rather good insight of other people. She cares deeply for Ryuuji and sees his potential. To support his dreams, she took on two jobs so that he wouldn't need to work to support himself through school. When he starts working at the bakery, she gets upset because she wanted him to focus on studying rather than working. She is known to Taiga as Ya-chan.