@monkey123 He takes good care of his property, yeah. The author can pretend this was a generic battle harem romcom all along but it really isn't, not when the initial premise was "asshole brainwashes girls into being his sex slaves".
All these "romantic" developments are just the brainwashing doing its job.
@Acolytus
Idk about that. It definitely can be argued that it started as that, but at somepoint, it stops being Stockholm Syndrome and just becomes genuine feelings. Stockholm Syndrome implies that their capturer is still hostile, apathetic, and/or abusive towards them. Or that they are being driven out of a "need to survive" instinct.
Stockholm Syndrome is a coping mechanism for dealing with being kidnapped or held as a hostage. She technically is not a hostage or was kidnapped. She approached him first, then lost.