Mogana Kikaijima is a first-year student at Hakoniwa Academy, later promoted to junior, and serves as the treasurer of Medaka Kurokami's Student Council. She originally belongs to the swimming club, where she is known as its ace, and she forms close friendships with the club's captain Yakushima and vice captain Tanegashima. All three share an intense obsession with money, driven by a common goal to amass enough wealth to fill a swimming pool and swim in it.

Kikaijima's fixation on money stems from a painful childhood. Her father abandoned the family due to debt, her mother fell ill from overwork, and every friend she had left her because her family had no money. These experiences lead her to believe that while people would grieve over a lost wallet, no one would care if she died. Medaka Kurokami challenges this worldview by expressing that she would indeed be sad if Kikaijima died, prompting a gradual reformation. Medaka recruits Kikaijima as the Student Council treasurer, making her the only council member who is paid for her work—three hundred yen per day. Even after Medaka's intervention, Kikaijima remains deeply money-oriented; shortly after joining the council, she charges Zenkichi Hitoyoshi for accidentally seeing her change, again for insulting her, and a third time for complimenting her figure.

Despite her lingering greed, Kikaijima develops strong relationships with her fellow council members, especially Medaka and Zenkichi. Encouraged by Yakushima and Tanegashima to form friendships beyond the swim club, she becomes a loyal and valued part of the team. She is classified as a "Special," a category of scholarship students with exceptional abilities in a specific area, similar to fellow council member Kouki Akune. Her primary abilities stem from her extraordinary lung capacity, which allows her to hold her breath for up to a month. She can also focus her voice into a powerful sound cannon, unleashing ultrasonic shrieks capable of destroying physical objects.

In the story spanning both Medaka Box and Medaka Box Abnormal, Kikaijima plays a supporting role within the Student Council, assisting in various conflicts including the Flask Plan and confrontations with the Party of Thirteen. Her development is marked by a shift from a cynical, self-serving attitude to a more balanced appreciation of friendship and personal worth, though she never entirely abandons her love for money. A decade later, she reunites with her former council friends. She has an infant son named Mogami, meaning "best," whom she considers her greatest treasure. She hands her son to Zenkichi, expressing hope that Mogami will grow up to have a strong will like his. Though she is married and no longer bears the surname Kikaijima, she continues to be called by that name at her workplace.