Death despises Necromancy and those who practice these art. The reason for this is, as you already said, that Necromancy denies the Souls of other beings their deserved rest, a fate that Death can’t allow to pass.
So, it might not that recommendable for your character to practice Necromancy. At least if you don’t want to land on Death’s bad side.
Don’t worry, I will keep the answer short and relatively spoiler free.
You see, Death (who would normally be adressed as Catherine within the game) would actually approve of the relationship between her daughter and the MC. As Sophia’s mother Catherine witnessed on first hand how the personality of her daughter changed during her years as a Soul Collector. It went even so far, that she feared that she would lose her sweet daughter entirely.
However, as I explained in one of my previous posts, by spending time with the MC Sophia would slowly discard her cold outer self and expose more of her real self. She would basically turn back to her real self because of her affection towards the MC. This is why Catherine would approve of a relationship between Sophia and the MC, because she believes that the MC might be able to “save” her daughter.
However, Catherine would still do everything she can to protect her daughter from the pain she had to endure through Tobias. So, while she might see you as the perfect partner for her daughter, she would still make it clear to you, that if you break Sophia’s heart you will experience a fate that would be far worse than death. As you already said: “If you break my daughters heart, I’ll break you in ways you can’t even imagine…” .
I hope I could answer your question with this.