I don't think it's possible that Seishin hates Toshio without even an ounce of affection though, as Toshio is pretty much the only person he actively seeks the company of since they were children. Like Toshio about him, Seishin also understands a lot about Toshio and I think the exasperation are only limited to trivial things? He feels annoyed because Toshio does something which he knows will annoy him, pretty much the only person he uses a little bit of sarcasm with ('he isn't from the parish families, not that I know all of the kids of the parish either'). Him showing annoyance when he knows Toshio is intentionally (but jokingly) picking on him, now that I think about it, I think that's understandable. I don't think Seishin has shown annoyance when Toshio is being serious, even when he yells at him when he's all distressed with the epidemic because Seishin gets that he's distressed.

The same goes to Toshio. Toshio may find some aspects of him unacceptable, makes him all but yell at him, but they never bring him to actually dislike Seishin. (I think? Waiting for later translations from you) If he were to go by his personal feelings, he won't hunt Seishin down even when he actively sides with the Shiki (implied in the novel!car scene). ((Considering how people usually act, these aspects of them tell so much I think. Romantic or not, I think they must really like each other. But that's kinda no news.))