Why is who doing what?
elliot aka the “brother” william
He's not obsessed with her he's in love with her. Like the situation is weird and wrong. 10 outta 10 would not recommend this as a pick-up technique. He's trying to make up for the death of her brother because he feels guilty. But the plan to look after her like the brother wanted got mixed up when he developed feelings for her. Also she's mentally ill and seeing things so her perception is skued. So we probably can't really trust anything we see as we're seeing it from her perspective.
Lol no he's not. He just had feelings. You need to read other truly obsessive characters to differentiate
he is sick and obsessed with her, why is he doing that