Elisheva

This feels more coming of age than conventional BL, which may be why it worked for me. Given how split opinion is, maybe it's worth it to throw my read into the mix? I found both of the leads believable, even if puppy Natsuo was a bit exaggerated. There's a lot going on under the surface.
Major spoilers below. I wish we had the option to spoiler bits out in reviews.

We see three sides of bullying in this (I'm including the classmates he meets again briefly, the ones who didn't change.)

Hiro went through high school in defensive mode and hasn't let it go. For all he's changed himself in appearance, learning a fashionable skill and developing confidence in that area (but not others), that hurting kid is still at his core. I don't think he actually wants revenge, certainly not in the way a few of the reviewers were after. More like seeing Natsuo again brought all of that back. Natsuo wouldn't go away so to protect itself, the scared, scarred, hurting part of Hiro's psyche came up with a plan of 'revenge'. And yes, it was never going to work because Hiro was never truly vengeful. It was also a way for Hiro to be near Natsuo and soak up his puppy energy, affection and enthusiasm without acknowledging it to himself.

Natsuo makes sense as a kid who didn't pay much attention in school. He could learn when he tried but he had no one at home encouraging him. (IIRC Hiro described their high school as being mostly delinquents - perhaps everyone had written the students off already and the teachers weren't really trying either.) Being gay, not the brightest bulb, with absent parents, he was very vulnerable and ill-equipped to cope with peer pressure.

I don't think I'm reading this into the story. There's a lot in this that's understated but still present. Maybe it helps if you watch enough coming of age? Japan makes some excellent CoA films, the best country in the genre IMO.

Hiro trying to force himself on Natsuo felt out of place to me. The whole tone changed so much, dark melodrama. The only way I can make sense for its inclusion is that the traumatised part of Hiro's psyche (which developed to protect him from previous danger and is still holding onto that purpose) felt severely threatened by the disclosure - but because the series is short and there was a lot going on (all of those side stories some complain about as 'filler' have purpose in developing the characters) it was abbreviated. I'm not in any way justifying that - if they were going to cut clear resolution short for time, it would have been better to drop this bit altogether and find another way.

Following this interpretation, perhaps Natsuo submitting later reflects his continued vulnerability to peer pressure. If so, these dots should have been connected in dialogue. He's grown but maybe not in this way. He's still very much a kid who needs to be loved.

So that's how I see it, a story of two teenage kids, lonely in different ways, now become young men and finding in each other what they each need to grow.