She is Phosphor’s dark reflection as a result of the curse, and her instinctual goal (like all Noreponies) is to absorb the life essence of her host, and replace her. Expression of bad emotions fuel them.
Much like Phosphor, she is extremely adept at alchemy and potion-making, though to a lesser extent. She is generally more mean and rude, and more easily gives in to greedy and wrathful impulses. Additionally, her colors are darker and more desaturated, while adding colors reminiscent of Phosphor’s father to the mane and tail.
Though Nitrous’ ultimate goal is to essentially kill Phosphor, Phosphor is hellbent on turning her into a sister. She’s one of the few that has realized that if they feed off of bad emotions… maybe good ones can make a difference.