In chapter 1, stalker is how she had described him to Asahi, staring at her whenever he sees her. He's been crushing on her for 3 years, she's clearly been aware of it and creeped out for some time before any of them know he's the drug stealing crush of her crush who he'd never met until chapter 1. Maybe it's just me but, being different years at school, I wouldn't assume they'd be friends at all, just someone she knows as her sister's friend, who might come to their house, apparently staring at her. Then once we're in the story, he confesses, gets rejected, finds out her crush is a girl and still keeps trying.
But she's divisive, I like her treatment of him because I think he is creepy, others don't so no sense in arguing.