Rinko Nikaidō (二階堂凛子 Nikaidō Rinko) is a character from Yudetamago's manga and anime series (Kinnikuman Nisei). Rinko is a friend and occasional love-interest of Mantaro Kinniku, as well as the adopted daughter of Mari Nikaido.
Rinko is the adopted daughter of Mari Nikaidō.  She attends Shirabara (白薔薇) Private High School, Class 2-A. In the original manga she is a kogal and debuts after the Hercules Factory Second Year Replacement Matches, but in the anime she is a more wholesome high-school girl who loves wrestling and first appears during the Tel-Tel Boy fight. Her two best friends are Keiko and Tamaki Maekawa and they are almost always seen with her, although in later arcs they have become more background and supporting characters, until they were almost entirely absent from the Ultimate Chojin Tag Tournament.
In recent arcs, Rinko has formed a close friendship with Jacqueline Muscle, and the two have been inseparable in the final arc of the manga. Rinko has shown a strong romantic interest in both Jade and Chaos Avenir, while Mantaro has shown a romantic interest in Rinko, and she is generally considered by fans - and characters within the anime - to be the girlfriend of Mantaro (even if she objects to this statement).
She refers to Mantaro as Manta (マンタ). In the Japanese version, she and her friends refer to Seiuchin (Wally Tusket) as Sei-chan (セイちゃん).
Rinko is Mari's adopted daughter. Mari found her as a baby on her doorstep and raised her from that moment onward.  Not much is known of her childhood prior to meeting Mantaro, although some flashbacks in the Ultimate Chojin Tag Tournament reveal some familial scenes between Mari and Rinko, such as learning to cook for the first time. We do also see some flashbacks in the manga, such as running in a three-legged race and participating in coming-of-age ceremonies.  It appears Rinko had a happy childhood, and she also prides herself on being tough and independent.
First Love (Filler Arc)
Rinko first appears at a speed-dating event, where Mantaro takes a romantic interest in her, but she rejects his advances by tearing up his card that asks for a date. Rinko later sees him rejecting a 'compensated dating' offer from a woman, and - after he is beaten up by the woman's boyfriend - Rinko comes to his aid and offers to take him home to rest.  Rinko is then caught trying to sneak back into her house after curfew, where is revealed she is a delinquent, and that she also resides at Suminoe Kindergarten with Mari Nikaido as he adoptive mother.  Rinko displays a strong love for her mother, and attends an all-girls school, which is revealed as Mantaro follows her in hopes of getting her attention. 
Rinko stands up to the bully Tamaki Maekawa, who is attacking Keiko for not giving her 'protection money', and and Rinko effectively saves Keiko from harm.  Rinko follows Keiko, who is forced into 'compensated dating' in order to protect herself from Tamaki, and saves Keiko from a dangerous man, only to be threatened with rape by what is revealed to be The Rigani.  Rinko is stripped down and taken hostage, forcing Mantaro to fight for her honour and to save her from The Rigani, and - as she cheers him on - she comes to respect him and his strength, after which she is finally rescued as The Rigani is defeated. 
HF Second Year Replacement Matches
Rinko shows an interest in Jade, to which Mantaro becomes jealous. Rinko appears to watch many of the matches and provide support to the competing characters.
Kajiba no Kuso Djikara Challenge
Rinko is told by her mother, Mari, to email Mantaro in Abarashi.  Mari thinks an email from a girl he likes will motivate him, but Rinko believes that more traditional methods like energy drinks would be better. Still, she sends him a suggestive photograph, with the private parts edited out with a black bar, which says she'll show him the rest should he win the match. The photograph - unknown to Mantaro - is not nude, but instead is merely Rinko in a revealing swimsuit. 
Choujin Olympics: The Resurrection
It is revealed that Tamaki and Keiko think Rinko is no longer fun, particularly in the past month in Mantaro's absence, and Mantaro - hearing this - believes she has a boyfriend.  Mantaro has escaped from his duties on Planet Kinniku to suprised Rinko on her birthday with a cake, which he lights up using his lantern and the power of 'Ultimate Muscle', and Rinko is finally shown to be happy. 
During the final round of the preliminaries, a three-legged race is announced.  In the anime, every Chojin is paired with a random person chosen by Ikemen, and in the manga, every Chojin is allowed to pick a contestant from the crowd.  In both cases, Jade is assigned Rinko (Rinko also shows romantic interest in Jade in the manga).  Mantaro shows jealousy, but Rinko is honoured and works very well with Jade, as she listens to him and his commands, and their co-operation allows Jade to win the preliminary race.  In the middle of the race, Jade gets into a fight with Ilioukhine, but the combatants stop so they can finish the race. Jade and Rinko finish the race third - Rinko then faints. 
[The Poison Six-Pack Arc occurs here in anime; see "Anime Changes"]
Demon Seed Arc
Rinko chastises Mantaro when he refuses to team up with Kevin, as she sees the need to save Meat more important than his pride, and throws an empty can at him in frustration.  It is Rinko's tears and emotional plea that later stops Mantaro and Kevin from fighting.  Later, she takes Kevin's arm and tries to get him to shake hands with Mantaro, but both teenagers object and continue to fight, which causes her to give up hope.  Rinko finds some hope again when she sees the Seigi Chojin drop everything to help their colleagues, as they chase after Kevin Mask and Mantaro in hopes of helping them reconcile. 
Ultimate Chojin Tag Arc
Rinko is a major character within this final manga arc. Initially, Rinko is a stowaway on the Kevin Mask Time-Ship, as she wishes to go back in time to see her mother and spend more time in her mother's presence, and she is taken under the care of Jacqueline Muscle, as the two become inseparable. At first, Rinko shows great distaste towards Chaos Avenir, as she sees him as an otaku and a pervert, but - over time - she is won over by his complex back-story, which reflects her own personal past. They have both lost biological parents, only to be adopted by strong and caring women. Chaos also shows a selfless and kind personality.
The two develop a potentially romantic relationship, with Rinko even dressing in cosplay for Chaos to inspire him, and when he dies it is Rinko who grieves the hardest and carries with her his closest possessions. Rinko spends much of the remainder of the arc mourning for his loss, as well as supporting Mantaro in his matches to avenge the loss of Chaos. Eventually Mantaro and Kevin Mask win the tournament, allowing the characters to return to the present timeline. The ending is abrupt, so nothing is known of what happens to Rinko after they return to the present time-line.
Rinko is introduced in the anime alongside Tamaki and Keiko, where she is seen to be a fan of wrestling and expresses a desire to watch matches. This grabs the attention of Mantaro. In the manga, Tamaki bullies Keiko into dating The Rigani for money, to which Rinko steps in to defend Rinko and prevent the exchange, and this leads The Rigani to kidnap Rinko. In the manga, he exposes her to her underwear, but this is changed in the anime due to Rinko's age, so that she is fully clothed. Mantaro saves Rinko and they become friends from that point.
Poison Six Pack (Anime Only)
After the Chojin Crown prelims are finished, a group of Akugyou Chojin known as the Poison Six Pack kidnap Rinko, Keiko, and Tamaki as hostages for the upcoming tag matches held around Mount Fuji. All three girls are held in a floating jester head, wearing revealing Tarzan coloured bikinis (these outfits are censored in the dub). Tamaki is the first hostage held in a glass capsule, hanging off a part of the jester head. If Team AHO lose a tag match, the girl held in the capsule become's the Poison Six Pack's slave.
During the first match, Terry the Kid and Jade vs. Dazzle and The Protector, Rinko and Keiko shout encouragement to a scared Tamaki, as Kid and Jade are being pummelled by the first two members of the Poison Six Pack. After Tamaki is freed, it is Keiko for whom the chojin fight to free. With encouragement from Rinko, Keiko and Gazelleman, Seiuchin dives into the lake to save his mother and sister. As Mantaro fights Baron Maximilian, Suguru uses Roxanne as encouragement, saying things like her being Mantaro's future wife, which irritates Rinko and makes her grow angry in her glass capsule.
Near the end of the fight, Mantaro nearly hits Rinko's glass prison, holding onto the ring ropes to set up a massive Muscle Millennium. With Baron defeated and Mantaro victorious, everyone is bewildered as to why Rinko's glass capsule hasn't been released. Baron, in his final moments, makes the jester head explode, sending Rinko flying over the edge of Mount Fuji's crater. With Rinko safely in Mantaro's arms, the two are still falling off the edge, and are saved by Team AHO. Both Rinko and Mantaro humorously knock Gazelleman back into the crater.
Rinko, who is supposedly about to kiss Mantaro, quickly makes Mantaro kiss Nakano. Mantaro is mad, but Rinko says he was giving her the creeps (according to the dub). However, as she whispers "Truth is, I love-" the jester head explodes and drowns out the last part of the her sentence. Rinko skips away as Mantaro tries to make her repeat herself.
- Shirabara High School Student, Class A - classmate of Keiko 
- Her nickname is Rinko-chan (凛子ちゃん)