Haruka Tanaka is the older sister of main character Taichi Tanaka, and she is a college student who lives with him. She previously attended the same high school as her brother, being a graduate of Shirahamazaka High School.

As the head of the Tanaka household, Haruka takes on the responsibility of looking after Taichi, especially given his tendency to be chronically late and his dedicated focus on the badminton team. Her care for him is a quiet but constant presence in his life. However, their sibling relationship has a casually humorous and somewhat blunt dynamic. Haruka is notably relaxed and at ease in their shared living space, to the point where she has been known to walk into her brother's room wearing only her sports underwear to enjoy the air conditioning, much to his embarrassment.

Despite this carefree demeanor, Haruka provides crucial insight into her brother's character. She reveals that Taichi has been playing badminton since the third grade, highlighting his long-term dedication to the sport and underscoring why a tough loss in a tournament is a significant emotional blow for him. In this way, she functions as a supportive but grounded figure in Taichi's personal life, separate from his school-based relationships with his friends in the choir and badminton club.