I think her love for him was not love at all (she was jealous of another woman getting close to Hakusei, she probably loves him). Seitetsu is her father, in a sense, that's probably why she's so sad. Now she's truly orphaned.
If the author is really committed to this being her first love, it's really shoehorned in. It just wasn't there until this arc. As if Hakusei's life didn't suck enough.
I think The author should have made her love him as a father and a teacher, since this feels a little forced, she even was jealous when blondie was Talking to the spy. Nonetheless, i love The one-eyed a lot