This is a one-shot with quite the synopsis, but it unfortunately didn't live up to its outlandish premise.
Story, 2: The entire relationship was formulated on emotional manipulation: one partner was consistently unfaithful to spur the protagonist into action, while the protagonist battles with himself to quite literally not kill his partner. The conclusion was also lackluster and didn't serve to develop either of the characters.
Art, 6: This was satisfactory. The lines were a bit bold and there were a lot of seemingly unnecessary shoujo effects on the panels, but I may just be unfamiliar with Yuki's style at this point. It should also be noted
that this is a doujinshi, so it likely isn't up to par with a licensed release.
Characters, 2: While I've discussed most of my thoughts on this aspect in the 'Story" section above, it could also be noted that neither character feels particularly believable. This may be due to the fact that one isn't actually human, but regardless, I was thoroughly underwhelmed with their motivations.
Enjoyment, 3: I only started reading this as a warm-up for Yuki's other, longer works. I don't plan to let this deter me from this goal.
Overall, 3: A very quick read that shounen ai fans may enjoy, but it unfortunately didn't thrill me in any way.