Sumireko Ichijou is one of the twin sisters and childhood friends of Nozomu Futami, having lived next door to him in their early years. In kindergarten, both twins declared they would marry him when they grew up. After Nozomu moved away, she and her sister felt lonely, prompting Sumireko to suggest buying astronomy books to continue stargazing, a shared interest with him. They later reunited when attending the same school. During elementary school, the Ichijou twins gained popularity, and Sumireko wrote her wish to become a bride in her graduation album.

Sumireko nurtured secret romantic feelings for Nozomu, concealing them even from her twin sister. By Episode 10, she acknowledged these feelings internally but delayed confessing. Her hesitation stemmed partly from envy toward her more athletic and outspoken sister, Kaoruko, traits Sumireko felt she lacked. This jealousy originated in childhood, where she often perceived herself as merely following Nozomu and Kaoruko during play, struggling to match their energy. A pivotal moment occurred during a Ferris wheel ride at Fukubuku Department Store; after Kaoruko proposed to Nozomu, Sumireko impulsively did the same, hoping to preserve their trio dynamic.

In Episode 12, Sumireko resolved to confess to Nozomu on Valentine's Day. She confronted Kaoruko about their mutual feelings, revealing her longstanding jealousy while learning Kaoruko similarly envied her due to their parents' frequent comparisons. They reconciled, agreeing not to hinder each other's emotions, and both ultimately confessed to Nozomu.

Sumireko dislikes touching fish, a fear stemming from kindergarten when Nozomu assisted her during a killifish-breeding activity. She enjoys baking and has made apple pies multiple times. Her personality contrasts with Kaoruko's, as she is less assertive but deeply introspective about her emotions and relationships.