
Can anyone remind me again who Catherine is?

Because he is not recognised as a prince, no power and no backing. The only thing Ethel and his mother has, is the fact that she's the emperors lover.
Before this, due to the empress and opposing factions, Ethel and his mother were also hidden away in secret. And even though he's brought back now (thanks to Cayenna bringing it up before), Rezef is the one with the most power (atm anyway).
Major spoiler below.
Ethel is actually the ONLY prince. Rezef isn't the emperors son (the empress had an affair with ML'S father - so bascially rezef and ml are half brothers welp). The emperor knows, hence why he hates Rezef but also because of his pride, he doesn't want anyone to know the empress cheated on him. The whole family tree is a mess tbh.
It’s people like Belezac’s dad that have no right in raising a kid.
The "he's just a kid, boys will be boys, he's just playing and got too rough" shitty parent that just let their kids get away with anything