yeah that's actually true. he hates her calling him brother (oppa) and refuses to recognize her as his sister because of his obsession with her. I say it's an obsession bc it's not really a crush as normal people behave but rather the obsessive type where he wants to keep her all for himself. It's funny bc penelope thought he didn't like her calling him that bc of his love for yvonne but that's not the reason why
[spoiler] the thing about his obsession with penelope is that it manifested into hate. Derrick doesn't just have feelings for her but he's obsessed with her so much so that he wants to keep her in this cage (so to speak) all for himself. This obsession with her began to manifest itself into hatred and thus the abuse began...yup he abused her bc of his feelings toward her. through the abuse though he still had feelings for her (this dude is crazy). He hated vinter and callisto being around her (more callisto bc he was the one that penelope was romantically involved with...vinter was an unrequited love). Don't expect character development from derrick though bc it won't happen. him and eclise are the trashy ml's of the group (ppl baby eclise too much bc of his personality).
This isn't explicitly stated and more of an inferred thing but so you know how derrick claims penelope does things for attention...he doesn't say it but he truly means that she'd doing it for HIS attention in particular.
also when penelope ran away from the duchy, she said goodbye to daddy eckart and while she was in the process of running away, she ran into reynold. Reynold didn't want her to leave bc he loves her (in a regular brotherly way...unlike some people) but he helps her escape bc he wants her to be happy/respect her wishes. Meanwhile derrick gets a bunch of the duchy's knights to chase after her and he eventually finds her and he starts begging her to come back (the audacity after the way he treated her) but she still manages to escape. Mind you this is all happening after eclise brings back yvonne and the poisoning/suicide attempt.
derrick is one crazy ass mf
Mann i hope her dad would beat the shit out of that black haired bro. How can he do that to our fl, she is your sister! Even tho she is not blood related, she is still your sis. How can you left her there to deal with that monster and jump over to protect some random girl you fucking bastard.