Seiko Ayase appears as a strikingly beautiful woman in the prime of her life, yet she is the grandmother of Momo Ayase. Her snow-white hair is usually tied in a large top knot, and her red, rectangular glasses frame her eyes. A chain smoker, she is almost never without a cigarette in hand. Despite her uncouth manner of speaking and phlegmatic demeanor, Seiko possesses a deeply selfless and empathetic nature. She readily takes in those in need and believes in sharing burdens, especially for the sake of children. Her youthful appearance often causes people to mistake her for Momo's mother, leaving them shocked when they learn the truth.