Rinko Nikaido, adopted daughter of Mari Nikaido, was discovered as an infant on Mari's doorstep and raised by her. Flashbacks reveal a generally happy childhood, including participation in events like three-legged races and coming-of-age ceremonies, fostering her self-reliant and resilient nature. She attends Shirabara Private High School, Class 2-A, alongside close friends Keiko and Tamaki Maekawa, though these friends gradually recede into background roles later.

In the manga, Rinko debuts as a rebellious kogal who smokes, appearing after the Hercules Factory Second Year Replacement Matches. The anime presents her earlier during the Tel-Tel Boy confrontation as a more conventional high-school wrestling enthusiast. Her personality blends high energy, assertiveness, and occasional aggression, characterized by directness and minimal tact. Despite this, she exhibits fierce loyalty to friends, prioritizing their safety even at personal risk. Her rebellious streak includes rule-breaking, and she religiously attends wrestling matches involving Mantaro Kinniku.

Rinko first meets Mantaro at a speed-dating event, where he expresses romantic interest; she rejects him by tearing up his date request card. Her view shifts after witnessing him refuse a compensated dating offer and defend himself against an aggressor. She aids him and offers shelter, revealing her residence at Suminoe Kindergarten with her adoptive mother. Later, she confronts bully Tamaki Maekawa to protect Keiko, cementing their friendship. This leads to her kidnapping by The Rigany, who strips her and holds her hostage to coerce Mantaro into combat. Her vocal support during the fight earns her respect for him, and she is rescued upon The Rigany's defeat.

Her relationship with Mantaro evolves from his one-sided affection to mutual friendship. While initially platonic on her end, she actively supports him, emailing a suggestive swimsuit photo (edited with a black bar) during the Fire of Inner Strength Challenge Arc to motivate him. She later surprises him with a birthday cake after his absence, signaling growing mutual care. Her romantic interests extend beyond Mantaro: during the Chojin Olympics: The Resurrection, she partners with Jade for a three-legged race prelimenary, displaying strong coordination and clear attraction to him. She intervenes in his match by throwing in a towel to prevent injury, though it proves ineffective.

In the Demon Seed Arc, Rinko mediates, chastising Mantaro for refusing to team with Kevin Mask to rescue Meat. Her emotional pleas and tears temporarily halt their conflict, and she observes the Justice Chojin uniting. This period initiates her close bond with Jacqueline Muscle; initially at odds, they become inseparable companions, often defending the Justice Chojin together.

The Ultimate Chojin Tag Arc deepens Rinko's development. She stows away in Jade's luggage on a time-ship to meet a younger Mari Nikaido and join Jacqueline. Learning of a prophesied savior Chojin, she assists Mantaro in seeking a tag partner and identifies Chaos Avenir as the candidate. Though initially disliking Chaos and dismissing him as an otaku, she agrees to cosplay for him if he partners with Mantaro. Her commitment intensifies when she cheers for him during a critical match. After Chaos is knocked into the War-Cube, shattering glass severely injures her ankle. She refuses hospitalization to prioritize his morale and recovery. This act, combined with her encouragement, forges a profound emotional bond. Chaos’s eventual death devastates her, triggering incapacitating grief.

Rinko’s appearance features brown hair (occasionally blonde in the Ultimate Tag Arc) styled in low pigtails reaching mid-back, brown eyes, and pale skin. Her wardrobe varies across the manga. Key relationships highlight her complexity: the dynamic with Mantaro matures into mutual respect; her friendship with Jacqueline becomes central in later arcs; and her connection with Chaos shifts from aversion to sacrificial devotion.