Haruka Saigusa, self-dubbed "Haruchin," is a student notorious for disturbances and rule-breaking, resulting in frequent disciplinary actions. She often invades other classrooms, especially Riki Naoe's, instigating uproars for amusement that lead to chases by the disciplinary committee. This behavior stems from a complex family background where she and her twin sister, Kanata Futaki, faced mistreatment within the Saigusa family. While Kanata seemed favored, both suffered, with Haruka targeted as the scapegoat. Her troublemaking serves as rebellion against Kanata and sabotage of the family's reputation.

Physically, Haruka has violet hair in two ponytails secured with forehead clips, blue eyes, and stands 163 cm tall. She wears the standard school uniform but distinguishes herself with prominent striped thigh-high stockings, more pronounced in anime adaptations. She is left-handed.

Beneath her rebellious exterior, Haruka shows loyalty to friends and skill in repair work. She maintains a part-time job in the school cafeteria primarily to use the oven for baking muffins and cakes. Academically, she displays talent in mathematics, participating in a televised math competition during *Kud Wafter*.

Her development centers on overcoming deep-seated resentment. Initially viewing the world as unjust, she harbored hatred towards her family and Kanata. Through interactions, particularly with Riki Naoe, she reckons with her trauma, gradually accepting her family's actions weren't born of pure malice. This journey culminates in her understanding that self-worth exists independently from family approval, allowing her to release her hatred and embrace self-acceptance. She expresses gratitude to Riki for helping her see the world's capacity for kindness and light.

In *Kud Wafter*, Haruka makes a cameo appearance in the math competition segment.