So the lack of nutrition explains why she's so small despite being 20 years old, but then why does she also seem to have the mentality of a toddler? I guess because she was apparently kept in isolation for her whole life and didn't get the chance to learn anything? Pretty fucked up
She was locked in a cage most of the time if that flashback she had when she first went shopping for a plate (ch74) is anything to go by. Hidden away so well that even the Demon King, her grandfather, didn't know she existed until now, and only a few succubus servants know of her.
20 years of solitude, malnutrition, confinement.... is excessively cruel for a child even if they're long-lived race.
Even worse, she would probably
still be caged up if Grandpa hadn't found out she existed and attempted to kill her. Mom Succubus only released her into the human world as a desperate hail mary.