They complement each others character:
The FEMALE LEAD is an adult but has a childish character looks innocent and naive but not too much because she knows the nightly adventure hahaha
(ノ≧∇≦)ノ
The MALE LEAD is also an adult but only in age while the body is of a child that also affects his mindset making him think and act as a kid at times, but coincidentally more reasonable to common sense than FL (￣∇￣")
Overall the temper of ML is reasonable for the fact thta he felt left behind by his peers, while FL's past is also a mystery I would love to have a flashback scene for that, FL is OP with a cute ML almost like a baby
(๑•ㅂ•)و✧
Demon King’s Doll Butler