This is a weird one. It was marketed as a horror BL while it was being serialised online (for free!), but at some point it got published by Kadokawa and the BL tag was dropped. Mokumokuren has also deleted all tweets using the 'horror BL' line. I think the general feel for it is 'bromance' which in Japanese fandoms is similar to what we'd call a 'queerplatonic' relationship. Nothing about the story has been changed though, so the tension and intense emotion Hikaru and Yoshiki feel for each other is still very much there. Other characters also say things like 'About that friend... I thought you're talking about your girlfriend' or Hikaru remembering his dad telling him to marry the person he likes as soon as possible because the mountain is lonely and would steal the person he likes if he's not careful--but he's looking at Yoshiki.
Basically it's a coming of age story, where two boys are coming into terms with the fact that they might have a crush on each other. But Hikaru is dead. He died in the mountains a while back, and the person who came back isn't Hikaru.
Hikaru loves Yoshiki, but he doesn't really understand his feelings because he can feel and remember everything the real Hikaru has felt and gone through. Meanwhile, Yoshiki feels guilty about acting like nothing happened and having fun with this new Hikaru, because the boy he loves is actually dead.
There's a feeling of dread and listlessness, and I think a lot of that has to do with the art style (very horror manga) and the way the sounds of summer (like cicadas etc) spill across the whole page and outside the panels. It's just really beautiful. The balance between childhood first love nostalgia and horror is so good. I'm looking forward to vol2!