Nikki is Wombatman's girlfriend. She was held prisoner by Mother Brain, who forced Wombatman to capture most of the N Team in exchange for her safety. With some help from Kid Icarus, however, Wombatman was able to rescue her.
In some scenes, Nikki is drawn with an animal-like nose instead of a human nose. Since her boyfriend is an anthropomorphic wombat, this is most likely done on purpose to make her appear less human and avoid implications of zoophilia.