Sumire Hasukawa tutored Kazuya Hasukawa during his middle school years, becoming the object of his romantic affection. After the death of Kazuya and Kazuhiro Hasukawa's parents, she developed a relationship with Kazuhiro, the older brother who became Kazuya's guardian. She married Kazuhiro, an event that deeply affected Kazuya and led him to live in the Greenwood dormitory at Ryokuto Academy.

Joining the Hasukawa household after her marriage, Sumire consistently shows care and affection for both brothers, using the nickname "Ya-kun" for Kazuya. Her presence in the home alongside her husband, who was also Kazuya's guardian, created emotional difficulty for Kazuya. This marriage and her role as his sister-in-law formed a significant emotional hurdle for him.

The birth of her daughter Midori with Kazuhiro helped facilitate Kazuya's emotional development. Midori's arrival aided Kazuya's gradual acceptance of Sumire's marriage and his eventual emotional transition away from his former romantic feelings. Sumire maintains a nurturing role, providing stability and care within the family. Her husband, Kazuhiro, works as the school nurse at Ryokuto Academy.