Tsukiko Tachibana, later Tsukiko Fujii after remarrying, is mother to Hina and Rui Tachibana. Her first marriage ended when husband Jou Mikimoto abandoned her for another woman, inflicting deep emotional wounds. The divorce plunged her into years of bitterness and distrust toward men; she openly voiced resentment about her ex-husband to her daughters, negatively shaping Rui's views on relationships. This bitterness eventually receded after meeting Akihito Fujii. His sincere commitment helped her overcome depression and heal, his supportive vow restoring her joy and emotional stability, leading to their marriage and forming a blended family with his son, Natsuo Fujii.

As Natsuo's stepmother, Tsukiko shows him genuine warmth and kindness despite no biological tie. She consistently prioritizes others' well-being, sometimes at her own expense. This self-sacrificing tendency led her and Akihito to mutually consider divorce out of concern their union strained their children, though they later reconciled. She actively nurtures household harmony amidst complex family dynamics, including tensions stemming from Natsuo's relationships with Hina and Rui. Physically, she possesses light brown to blondish hair cropped at chest length with bangs, green eyes, and a typically cheerful expression that shifts to visible anger upon mention of her ex-husband.

Her background reveals resilience in rebuilding after betrayal. The name Tsukiko means "moon child," her maiden surname Tachibana signifies "orange" or "tangerine," and her married surname Fujii derives from "wisteria" and "well." Her development centers on transitioning from resentment to emotional renewal through Akihito's support, cementing her role as a stabilizing family force.