Yukako is introduced in the midst of a date with Koichi Hirose,[3] initially acting demure and sweet around him. This visage quickly dissipates when she reveals her bad temper and obsession with him, which occupies the majority of her initial role in Diamond is Unbreakable. Yukako's most recognizable character trait is her obsessive love for Koichi, which she explains as appreciation for men with potential rather than perfect people, whom she considers boring, and admits outright to becoming obsessive about anything she likes. Her love takes an overtly psychopathic turn when she decides to kidnap him and attempt to brainwash him into reciprocating her feelings, as well as forcefully training him into fitting an idealized picture of himself.