Mari Nikaido is a Yamato nadeshiko-type that was Kinnikuman's original love interest in the series for the first three story arcs. The anime on the other hand, kept her as the love interest for the entire series throughout the 1980's and 1990's. While Mari plays a larger role in the anime, she is still a strong supporting character in the "Kinnikuman" series, and even becomes a main character in a revival arc during Kinnikuman Nisei. Mari's daughter is Rinko Nikaido.
Mari is first introduced cycling to work.  Mari recognises Kinnikuman as the 'man always beat up by monsters', and introduces herself as the manager of Suminoe Kindergarten (a preschool). Mari invites them to visit any time; in particular Alexandria Meat, who is her friend. The children misbehave and tease Kinnikuman, so Mari apologies to him and applies first-aid.  A monster called Franky attacks, which Meat manages to defeat when Kinnikuman is unable, and - with great gratitude - Mari kisses Meat, making Kinnikuman jealous in the process. 
Mari also is seen taking the children on a field trip.  When Meat gets motion sickness, she allows him to rest his head on her lap at the front of the bus, and Kinnikuman - jealous - swerves the bus on purpose (as its driver) to make Meat feel worse. The bus gets attacked, so Mari takes the children to safety. Later, when Kinnikuman is in hospital, she brings the children to visit him. 
Kinnikuman also takes Mari and the children on various trips, such as to the funfair  and to the beach.  At the beach, a monster comes and rips the clothes from Mari, but Kinnikuman saves her and covers her with a towel to protect her modesty.  When the Seigi Chojin take their beloveds on a date, flying through the air, Kinnikuman chooses Mari - Natsuko sees Mari and Natsuko accuses her of deceiving men for her own gain.  This causes Mari to weep. This concludes Mari's part in this arc. 
Mari continues her rivalry with Natsuko, as she watches the matches from the crowd.  The two exchange insults about each other being 'loose'/'fickle', which leads to visual puns based on the Japanese, where they literally float by their posteriors. Together, they celebrate Kinnikuman's victory.  Mari appears very little throughout this arc, but is often seen watching the matches. The last appearance of this arc is saying farewell to Kinnikuman, as he leaves for his American Tour. 
When Bibimbap arrives, Mari feels overshadowed.
Mari gives up on her romance with Kinnikuman, as she goes overseas to study.
First Love (Filler Arc)
In "Kinnikuman Nisei" Mari has returned to Japan and has an adopted daughter named Rinko who is the love interest of Mantaro Kinniku. Rinko is Mari's adopted daughter. Mari found her as a baby on her doorstep and raised her from that moment onward.  A series of flashbacks reveal some familial scenes between Mari and Rinko, such as learning to cook for the first time, running in a three-legged race and participating in coming-of-age ceremonies. 
Mari is first seen watching as Kinnikuman is afflicted by the Devil's Hourglass. 
Rinko tries to follow Mari, but loses her in the crowds. Chaos Avenir takes Rinko and her friends to the Gakincho Orphanage, where Ms Mari is playing a piano for the children, and Mantaro proceeds to flirt with her until Rinko interrupts.  Ms Mari cooks for the orphanage, while Sister explains that Ms Mari is a kindergarten teacher that volunteers to help with the children.  Rinko shares a room with Ms Mari, where they bond, and Mari explains that she is happy in life, before they proceed to sleep. 
The following day, Kinnikuman is on the news struggling with the Devil's Hourglass.  He is agonising pain, but the pain is eased any time Ms Mari looks to the screen, which causes Rinko to suspect that Ms Mari is the "Key Person" that can break the curse of the Devil's Hourglass.  Later that evening, Rinko and Bibimba beg Ms Mari in a nearby park to save Kinnikuman, and convince her she is the key person. 
While Kinnikuman later rests in his dressing room, they bring Ms Mari to him in hopes of breaking the curse.  It appears that the Devil's Hourglass is gone, once Kinnikuman and Ms Mari leave the waiting room, but Kinnikuman later relapses on the ring.  It is revealed that Ms Mari was willing to hug Kinnikuman to break the curse, but he loved Bibimba and refuses to betray Bibimba by hugging anyone else, and would rather endure the curse than to betray his partner. Bibimba thanks Ms Mari for her honesty. 
(More to Come)
- Theme Song
- "Catch Me, My Hero" by Kaori Kumiko
- Titles: Suminoe Preschool Daycare Worker (Principal in Nisei)