Mari Nikaidō, an Earthling from Japan, serves as a teacher and manager at Suminoe Kindergarten. She first meets Kinnikuman during a monster attack, recognizing him as a frequent target of such assaults, and subsequently develops romantic feelings for him, becoming a recurring presence in his early adventures. Demonstrating compassion and responsibility, Mari cares for children during field trips to locations like beaches and funfairs, administering first aid when necessary. Her interactions with Kinnikuman include bouts of jealousy, notably when Alexandria Meat receives affection from her, such as a post-battle kiss. Mari also contends with rivalry from Natsuko Shōno, whose accusations of deceitfulness cause her emotional distress.

Her role shifts over time: she spectates at the 20th Chojin Olympics, trading insults with Natsuko, but her romantic ties to Kinnikuman wane after Bibimba's arrival overshadows her. Feeling displaced, Mari abandons her pursuit of Kinnikuman and departs Japan for overseas studies.

Years later, Mari returns in the sequel series, adopting a daughter named Rinko Nikaidō after finding the infant on her doorstep. As Rinko’s adoptive mother, she raises her affectionately, sharing activities like cooking lessons, three-legged races, and coming-of-age ceremonies. Mari resumes her education career as a principal and volunteers at Gakincho Orphanage, playing piano for children and assisting with meals. She maintains a close bond with Rinko, comforting her during emotional moments and expressing contentment with her life.

Mari becomes indirectly entangled in supernatural conflicts when Kinnikuman is cursed by the Devil’s Hourglass. Rinko identifies her as the potential "Key Person" to break the curse, observing Kinnikuman’s pain eases when Mari views him via screen. Initially reluctant, Mari agrees to help and meets Kinnikuman. The curse briefly lifts after their encounter but resurfaces. Mari offers to embrace him to fully break the curse, but he refuses to betray Bibimba, choosing to endure the torment. Bibimba acknowledges Mari’s sincere willingness to assist.

Her appearance varies across adaptations: the manga depicts her with black hair just above her shoulders, a four-parted black fringe, and bright blue eyes. Early anime iterations show wavy brown hair to her shoulder blades, a three-parted fringe, and dark brown eyes, typically wearing a red button-up shirt, peach skirt, and red shoes. The sequel series redesigns her with blue hair above her shoulder blades, an unkempt fringe, and black eyes, often dressed in a light blue button-up shirt with lighter cuffs and collar, dark blue denim shorts, and occasional red glasses.