She plays a much larger role in the Light/True paths of the story than she does in the Dark path, but the truth behind her is mentioned in both: Monnet is really an Ancient Dragon. She and the rest of her tribe have become pawns of Regal's as a result of Jude's machinations for the Spring War, and she harbors deep hatred towards both of them as a result.
Monnet has dark gray skin that is even darker than most deities', causing some who see her to remark that she looks like a "real demon." This is because she is a mix between an Ancient Dragon (who have dark gray skin/scales) and a human, which allows her a human form when Dragons normally don't have such an ability. There are dark red-purplish vein-like markings on her face, neck, and arms.
She has wavy, light pink hair that reaches down to her knees and is kept behind her by the helmet she constantly wears. Her light blue-gray eyes match the teardrop-shaped markings in between her collarbones and the accents of her outfit. Most of her outfit is black armor, along with a strip of cloth that extends from her breastplate to form something of a skirt.
Though Monnet seems to come across as apathetic to the situation around her, it is largely because of her disdain towards humans and her distaste for having to serve one. She has a deep pride in her clan and is very protective of it, which is why she despises Jude and Regal.
Monnet is a rather brutal and pragmatic: She planned on giving Jude a gruesome death and make his head bow in reverence to the clan he abandoned for his treachery. She also wastes no time in killing Regal once she realizes that Jude's magic on the Dragon's Nest has faded, even poisoning her blade so that magic spells couldn't heal him.
Despite her hatred of humans, because Eirudy, Adrian, and Johannes help her retrieve the Dragon's Nest, she decides not to kill the rest of the humans after landing a killing blow on Regal. However, after finding out about Johannes' own treachery, she decides that after she kills him, she will go and terrorize the other Villages.
When Eirudy confronts her in post-game, she explains that she hates anyone that she deems a threat to her clan. Despite Eirudy defeating her, she claims that she will one day become stronger than Eirudy and will kill her and all the humans.
When Monnet is fought in Chapter 5: The Last Cup, she has 41,240 HP and drops 380 Vitality after her defeat.