What she was like before her imprisonment is unknown, but it is safe to assume that she was famous and well-beloved for her skills, as well as charitable enough to make a potion for the Bog King.
She enjoys teasing people, laughing as Sunny when she made him scream and even mocked him to shape-shifting into him to show him his frightened face.
After years of imprisonment, it can be safely assumed that the isolation has mildly eroded Sugar Plum's sanity to where she has become partially unstable. She is shown to be somewhat catatonic and unresponsive until Sunny showed her the primrose petal. She is prone to mood swings - switching gears of whether or not to make Sunny his potion with the promise of her freedom - and has become rather spiteful of the people who imprisoned her. She becomes desperate and is militant to the idea of expressing the truth behind Bog King's faux pas, using what went wrong as an aesop to those who misuse the potion.
Despite this, she seems to let go of her grudge as soon as Sunny frees her (the second time), saving Griselda from the crumbling castle and keeping Bog from killing Roland when he dusts Marianne with the potion.
Powers and Abilities
Unlike every other fairy seen, Sugar Plum is the only one seen capable of wielding actual magical powers.
She is capable of flight in spite of not having wings. She has the power of shape-shifting with seemingly no limits (from shape-shifting into full people to manifesting objects like drums and pedals) and can create miniatures of herself to manage tasks called "plumettes". While she can shape-shift into virtually anything, she cannot mimic voices accurately, the only one close being the Imp.
Her most infamous ability is the power to synthesis a love potion from a primrose petal, with complete understanding of how the potion works, and by extension about the ways of love as a whole.