@Kllick
I'm fairly certain her age has never been stated, however she's suggested that she is older than the demons seem to think she is. She's probably not a child.
Demon monk is (demon) clergy. He should just perform a (dark) ceremony and enslave/marry the princess in secret the next time he resurrects her. No more issues.
@MesaOracle look me in the eyes and say that demon monk even capable of plucking princess hair.......
He is spineless boy in love, maybe even his first love..... in 400 years ... or so....
All they need to do is sleeping in communal dorm, problem (kinda) solved....