I hope this doesn't end badly but I think it's probably going to

 I am now current on The Summer Hikaru Died and I'm really enjoying it so far. I think it does a really good job exploring the tension inherent in its premise. Which if you didn't know is that a teenage boy named Yoshiki discovers that his best friend (who I'm pretty sure it's implied he had a crush on) has been replaced by some sort of horrific, otherworldly creature, but he still wants to be close to it because the creature is so very like his vanished best friend, so he does his best to continue on as normal in the wake of this revelation. 
The not-Hikaru is inhuman enough to feel genuinely alien and not just like a human who can do some freaky body horror stuff. He's also characterized with a really compelling mix of danger and almost childlike innocence.
The dynamic between him and Yoshiki kind of reminds me of Ringo and Ecolo, though it's darker and more fucked up. I guess I'm just a fan of the complications and tensions inherent in the dynamic between a misfit teenager and the eldritch abomination that's imprinted on them like a baby chick.