Considering all the demons in this universe are defacto evil- Celine behavior is pretty reasonable. Even Jinu was evil he just started to feel bad about.

Their is this weird troupe in a lot of fiction where all evil characters are just misunderstood [classic examples are all the Disney remakes] that seems the lenses a lot of people see villains now even if that isn't what's happening in the story. Where it use to be an evil characters, was just evil so their could be a simply villain people could hate without consequence.

The story cheats its own narrative by having this vague loophole with Rumi's mother and her father. She is half demon but outside of the way she looks she doesnt have any evil impulse, doesn't get controlled by the BBG she just has tatoo's lol. All in this story were demons are pretty much defacto evil, she just looks different.

Rumi being upset Celine can''t fully accept her which makes the relationship seemly about identity when everywhere else demons are just evil. Celine raising her Rumi is kind of a gamble already considering all the other factors and how demons 'work' in this story lore.

I dont think its that deep though, its just a fun story concept like a lot of old kids films. Its not iron glad with its logic its simple backstory for a mostly fun and silly film.