They have a better relationship in the novel and yes he is very obsessed with her
The yandere trope is only romantic and working when the partner of the yandere love him back and kinda don't mind being controled after all. If feelings were not reciprocated or the partener finaly wanted to break the relationship it would quickly turn into a very disturbing tell. Well, in the novel he never crossed a red line (be it bbecause he decided to respect her wishes or because he didn't have to yield) so who knows for this one.
As a novel reader i assure you..don't worry and just continue to read..you'll eventually see that they actually have a healthy relationship.ლ(´ڡ`ლ) ٩(๑❛ᴗ❛๑)۶
I'm kind of getting scared about their relationship... I mean, he is becoming obsessed with her in a not very healthy way. She can't do anything without him knowing and even when she tells people about something they don't respect her and tell the duke (yes, I'm talking about the buttler since she told him the duke already knew and he still told him like her word wasn't valid). You can say is romantic but for me it's turning out pretty creepy.