Nikki is a pink anthropomorphic cat with a cream-colored face, first appearing in the 2012 series Shimajirō no Wow!, where she moves to Challenge Island and becomes a close friend of Shimajiro and the others. She replaces the earlier character Ramurin as one of the main preschool-aged protagonists. Nikki lives with her parents, an older brother who is also an excellent athlete, and her grandmother. Her mother works at a publishing company and is often busy, so Nikki helps with household tasks and shows a responsible side.

Nikki is portrayed as a sporty, energetic, and confident girl who excels at running, jump rope, tree climbing, and riding bicycles. She is described as caring and always willing to stand up to bullies, and she often takes on a big-sister-like role among her friends, looking out for them and encouraging them. Despite her strong and outgoing nature, she has a sensitive side; she can become discouraged, such as when she learns a toothbrush fairy story might not be true, and she is afraid of swimming, which is one of the few physical activities she struggles with. She is also secretly afraid of heights, even though she is capable of flying.

Her primary motivation is to enjoy active play and help her friends whenever they face a problem. She is always eager to join in adventures and challenges, and her determination often inspires the group. Over the course of the series, Nikki learns about perseverance, patience, and the importance of relying on others, gradually becoming more thoughtful in her approach. Notable abilities include her athletic talent, particularly in land sports, and her natural agility. Her energetic, can-do attitude makes her a key member of the friend group, balancing out the more cautious or dreamy personalities of her peers.