Haruno Kasai works as a proofreader who later becomes an editor. She enjoys discussing relationships and gossip with colleagues but has no romantic partner, preferring reading books. She openly admires the poet Kirameki Mikawa and thrills at becoming his editor. While her demeanor seems somewhat youthful for her age, she reveals inner strength during crises.

Her role proves pivotal during a work visit to Kirameki's home. Amidst revelations of Fumika's traumatic past and dissociative identity disorder stemming from Kirameki's abuse, a confrontation erupts where Kirameki shoots Fumika. Haruno intervenes to halt further violence, physically restraining Kirameki when he breaks free and preventing Fumika from attempting a killing.

Haruno maintains a supportive presence towards Fumika and others. Following the events, she visits the hospitalized Fumika Mikawa alongside Kaname and Natsuka. Her earlier observations of Kirameki's unusual behavior foreshadow this tense confrontation, where her actions help de-escalate the conflict.