I feel like Novel Seishin is a lot different than manga/anime Seishin. Anime/manga Seishin seemed like a trusted adult figure on the surface; mature and soft-spoken, he saw his suicide attempt as something foolish that he did when he was young, something he didn't really understand himself. He seemed wise and somewhat weary, not wanting to go along with Toshio's hot-blooded idea of there being vampires in the village (took him long enough amirite) and doesn't want to jump to conclusions. Which makes it all the more jarring when he totally flips out with Sunako, his world-view is suddenly shaken, and the idea of him being wise and introspective is shattered because his decision seems really immature.
However, novel Seishin is someone who I can see turning away from his village, rejecting the ones who rejected him. He still seems unstable, kind of brittle and emotionally removed from the village. So he's kind of buried within himself? It's very rare that we see him responding to outside stimuli apart from what is expected from him by the village (like with the broom- he seemed creeped out and very human at the time). So he doesn't seem rounded or confident in his position as he does in the manga/anime. I can easily imagine this version of him flipping out, unable to decide what's right and what's wrong.