After seeing so many episodes with PB along with Holly Jolly Secrets..
I think PB could be Betty
1. Same body shape
2. Betty looks like a scientist
3. PB doesn't age a bit
4. Imagine Betty without glasses and have really long hair.
5. Ice King chose PB as his fav princess.
6. Simon aka IK refers Betty as a princess.
7. PB's real name used to be Betty but change it to Bonnibel.
Who knows maybe I'm right or maybe I'm wrong. Anything could happen. Sometimes, there are something can happen without an explanation.
Don't hate it or any hatful comments, it's just my theory.