Yasuko is Ryuuji's thirty-three-year-old mother, though she refers to herself as eternally twenty-three; she has large, F-cup-sized breasts. Soon after Ryūji was born, her husband died which left her to raise her son by herself. She works at a hostess bar named Bishamonten Kuni where she goes by the name "Mirano" and single-handedly supports her two-member family financially. However, she has a childish personality and since she works, she entrusts the household chores to Ryuuji. She instantly takes a liking to Taiga and already regards her as a member of her family. (Source: Wikipedia)
Later in the show, Yasuko reveals that Ryuuji's father didn't die. He basically abandoned the two of them, leaving Yasuko to raise their son by herself.