Nyakkii Momoyama, called Nikki in English, is a pink anthropomorphic Persian cat. She features a beige face, pointy ears with lighter inner ears, and a forehead tuft of hair, plus a white muzzle and brown nose. Her typical outfit includes a light grey long-sleeved shirt under a purple vest, turquoise shorts, and purple sneakers with white sections accented by pastel purple stripes. For summer, she switches to a white t-shirt with purple stripes, a dark blue denim skirt, and shorter white socks. She occasionally dons a brown cowboy hat. Earlier designs showed her in a yellow and grey dress with light grey shorts underneath and black flats.

She attends Challenge Kindergarten with Shimajiro, Mimirin, Torippii, Zouta, and Kikko. Nyakkii embodies a tomboyish spirit, combining a hard-working attitude with sensitivity. Fiercely competitive, she dislikes losing and may cry after saying something harsh during contests. Despite her toughness, she struggles to confront Zouta's bullying. She actively opposes bullies at school and deeply values friendship, often joining group activities like soccer and other sports.

She excels at physical pursuits such as soccer, jump-roping, and climbing trees but is notably poor at swimming. Her favorite food is grapes, and she avoids both hot environments and cold places. She shares her birthday on April 4 with Hana-chan and Gao-Gao, making her zodiac sign Aries, and has blood type A.

During adventures, Nyakkii aids in practical tasks, like repairing a wooden bridge on Challenge Island, and joins quests to save magical realms such as the Glittering Kingdom. On these journeys, she answers calls for help alongside friends to tackle threats like disappearing inhabitants. She replaced an earlier character named Ramurin in the series.

Her surname "Momoyama" comes from "momo" (peach) and "yama" (mountain), while her given name "Nyakkii" incorporates "nya" (a cat's meow sound) and "ki" (yellow). Nyakkii has no siblings.