Nimona is the titular protagonist and is first introduced as a shapeshifter with a mysterious background. The narrative depicts her as a young white girl with short red hair. Throughout the story, Nimona’s hair symbolizes her evolution. Her hair is later purple, and it becomes increasingly shorter. She usually wears a short dress over what appears to be a chainmail shirt.

Nimona first approaches Blackheart because she admires his work as a villain and wants to become his sidekick. She is very enthusiastic and has a lot of energy, but she can be easily distracted. However, what first appears as a carefree, impulsive attitude is gradually revealed to be a much more destructive nature. Although she has good intentions, her moral compass is ambiguous, and she often reacts aggressively when she feels threatened. Blackheart is the only person she trusts, first as a mentor and then as something of a father figure.

Nimona tells Blackheart that she is the sole survivor of a raiders’ attack on her village and that an old witch gave her shapeshifting powers. The narrative later reveals that she made up her origin story. A flashback sequence shows that Goldenloin took young Nimona from her village to the Institution after she killed the intruders with her magical abilities and scared the other villagers.