Reverse mood swings are enough, i know... but have you ever payed closer attention? she acts mean...grumpy, playful, cute, happy, anxious, and many other ways... but..she can be childish like a young one...or even moody like a teen....but strict as and adult...

but how?? she died at 17

...

the answer? when Pitch killed her and brought her back with her powerful gift, he gave her the feelings of the life stage of a regular person... She can be playful as  a child, moody or snotty like a teen, and serious like an adult