Ok let's start with...

Name:Petals Dierdre (not canon. Also Chara and Buttercup's last name)

Likes: monsters, Chara, cats, reading, writing, drawing, love, and family.

Dislike: humans, blood, hate, darkness, and loud noises.
Fears: loneliness, the dark, large dogs, blood(this is for a reason), and abandonment.

Looks: long blonde hair, bright lavender eyes, porcelain skin, and a small scare above her right eye.

Past(sort of I'm not spoiling the 3rd book): she was found by Chara and Buttercup's parents. The parents wanted Chara to "train" with her, as Buttercup did with Chara. Chara refused thus making the parents hate Petals and shun her. They locked her in a closet so she could only leave when they wanted her to. Buttercup became softer because of petals, but she was still the favorite. Buttercup and Chara liked when Petals sung them a lullaby before bed cense they're parents never did.

Info on family: parents: where both involved in the human monster war. They where assassin's so they tried to train Chara and Buttercup to be assassins as well if the monsters ever escaped. (They made them practice on the younger sibling. This is why Chara didn't like buttercup as much.)
Chara: we should all know this 😒
Buttercup: now she loved Petals but knew she wouldn't survive a day outside so she forced Petals to kill her. Very popular with adults, and other kids. Died of "sickness" (explained later).

Um I think that's all I can say for now with out spoilers. Oh there was a few hints for later in the book 😏 I dare you to try to pice it together.

My drawings: