Grief, body horror and (very debatably IMO) mild gore, supernatural horror (ghosts/"impurities", otherworldly entities), death I suppose though that one's a given

The titular Hikaru went for a walk in the woods, died, and something came back wearing his body, something that doesn't quite know how to be human or mortal; "Hikaru" returned, to ensure that his best friend, Yoshiki, would not be lonely. Romantic tension (and tension of a less pleasant kind) ensues.

Yoshiki and "Hikaru" have an absolutely great dynamic- their relationship may not be fully healthy, but their intentions seem to both be in the right place, as are their deep feelings (Yoshiki's for Hikaru and increasingly for "Hikaru," and Hikaru and "Hikaru's" own true feelings) and honesty in this regard, despite the latter "Hikaru" being effectively an imposter of sorts. (it's complicated, but these are increasingly distinguished separately and I really like that process as well)

"Hikaru" is, while not quite a blank slate, extremely new to the concept of even just "being" in the sense that mortal creatures and individual organisms(?) do. They've had to learn (and Yoshiki has had to confront them about) the value and significance of life and death; they've increasingly established boundaries, and they approach the world with a liveliness and curiousity that is really cute (the original Hikaru was also lively FWIW, but "Hikaru" is experiencing everything anew even if they retain the memories).

Yoshiki, on the other hand, is both grappling with his grief and loss of Hikaru, while finding comfort (and discomfort alike) in his imposter, and in guiding them through a new world or state of being. He's finding his resolve and moral/ethical backbone interacting with "Hikaru" (very blue-and-orange morality dynamics, though they're learning), he's experiencing what could be described as a rocky but determined romance and queer experimentation at the same time "Hikaru" is being introduced to notions of attraction and desire (beyond instinctual desire to consume).

Their pairing in so many ways should not work (or rather would be usually destined for a tragic end). From the start, there have been several points where by all means it should have met such an end. But their determination and willingness to meet the other where they're at and gradually be understanding with the other is both fascinating and something I'd feel optimistic for (and interested in seeing play out further).