Rinko Nikaido, adopted daughter of Mari Nikaido who discovered her as an infant on her doorstep, experienced a generally happy childhood marked by learning to cook, participating in coming-of-age ceremonies, and developing self-described toughness and independence. She attends Shirabara Private High School, Class 2-A, often seen with best friends Keiko and Tamaki Maekawa, though their closeness lessens later as Tamaki and Keiko find her "no longer fun" during Mantaro Kinniku's absence.

Energetic, assertive, and direct, her personality borders on aggression with a rebellious streak involving rule-breaking and smoking (original manga). A dedicated wrestling fan, she frequently attends Mantaro's matches and exhibits fierce loyalty to friends, prioritizing their safety even at personal risk. This loyalty drives actions like defending Keiko from bullies or physically and emotionally intervening to stop fights between Mantaro and Kevin Mask.

Physically, she possesses brown hair—sometimes blonde during the Ultimate Chojin Tag arc—styled in low pigtails reaching mid-back, brown eyes, and pale skin, with outfits varying across the series.

Initially, her relationship with Mantaro Kinniku is one-sided: he harbors romantic feelings while she sees him as a casual acquaintance. This evolves into a platonic friendship of equals where Rinko supports him, such as emailing a suggestive swimsuit photo to motivate his victory in the Fire of Inner Strength Challenge Arc. She forms an inseparable bond with Jacqueline Muscle after initial friction, especially in the manga's final arc. Her dynamic with Chaos Avenir shifts from dislike—dismissing him as an "otaku"—to a profound connection after witnessing his maturity and talent. Shattered glass injures her ankle during his match while cheering; she refuses hospitalization to keep supporting him. His death later triggers intense, debilitating grief.

Key story arcs define her journey. In the "First Love" debut arc, she rejects Mantaro's advances but later aids him after a confrontation, revealing delinquent tendencies and her home at Suminoe Kindergarten. She defends Keiko from extortion and rescues her from a dangerous "compensated dating" situation, leading to her being taken hostage by The Rigani, requiring Mantaro's rescue. During the Chojin Olympics: The Resurrection, she participates in a three-legged race with Jade, showing romantic interest; they cooperate effectively to place third. She later throws in the towel to save Jade during his match against Ricardo, though he is severely injured. In the Demon Seed arc, she mediates between Mantaro and Kevin to prioritize rescuing Meat, using tears and verbal appeals. Her involvement peaks in the Ultimate Chojin Tag arc: stowing away in Jade's luggage on a time-ship to meet a young Mari, joining the New Generation's efforts, and searching for a prophesied chojin alongside Mantaro and Jacqueline. After identifying Chaos as the candidate, she reluctantly agrees to cosplay for him if he partners with Mantaro. Glass from the War-Cube injures her during Chaos's match, but she insists on remaining at the venue to encourage him. Her background, relationships, and actions depict a character defined by loyalty, evolving connections, and a willingness to endure physical and emotional hardship for others.