
Great conclusion!
I'm still a bit confused because of his brother... I hope we will get some more information in the future, even if he's already dead.
But it's exactly like you said... they need to communicate because it's just a bunch of misunderstandings and with every time they hurt each other it somehow gets more difficult so get back together... ( ̄∇ ̄")

The brother.
They have mirror moles. Look at the moles and their placements on their faces. I swear they look like mirror twins.
It definitely looks like the dead brother. And it looks like the dead brother is the one who haunts mc and teels him he will never be blondie.
Ch 2
https://www.mangago.zone/read-manga/backlight_1/uu/br_chapter-188635/pg-1/
This is the best image of the kiss. Look at the moles in the second and third images with the kiss. It shows the dead brother and why I think it's him who kissed blondie.
Ch 3
https://www.mangago.zone/read-manga/backlight_1/uu/br_chapter-188639/pg-1/
Look at who is tormenting mc. It's the dead brother.
Look at the mole on Hae-Yool's face, it's not a reminder of blondie but his brother. The one who tormented mc and destroyed their relationship.
Look at that image of the two brothers. Look at the dead brother's face. Looks like the guy in the kiss scene, right?
He thinks blondie and Hae-Yool are dating because he thinks blondie is looking for a replacement to his dead brother. Because of how similar the two look.
Ch 6
https://www.mangago.zone/read-manga/backlight_1/uu/br_chapter-188653/pg-1/
This is the moment we realize that the guy who has been haunting mc is blondie's dead brother.
And that the dead brother had a Dr. Serious health issue?
Ch 8
https://www.mangago.zone/read-manga/backlight_1/uu/br_chapter-190578/pg-1/
Another shot of the photo of the brothers.
Plus we get hints on mc's complicated past and how he is considered Mr. Cha's bastard son. But is that Mr. Cha referring to hyung? Or his father? Or is it just rumors to cover up the truth about mc's runaway past which we get hints of in ch 26.
Ch 9
https://www.mangago.zone/read-manga/backlight_1/uu/br_chapter-191430/pg-1/
The moment we realize the brother is dead and it seems blondie has negative emotions for him when in flashbacks they were closer.
Also, the grandma drops this big bomb about not being related and guilt for the brother's death.
Re-reading this again...does this mean the dead older brother was not a blood relation? And...did the older brother commit suicide? Not health issues?
I'm unsure of this but feels like chekhov gun moment and at some point in this story we will find out what this means.
Ch 10
https://www.mangago.zone/read-manga/backlight_1/uu/br_chapter-192475/pg-1/
Blondie also health issues. And we are shown that mc and BOTH brothers used to be close.
Look at the back of the head of the guy on the couch in the flashback. Looks like the head of the guy from the kiss, right?
Ch 11
https://www.mangago.zone/read-manga/backlight_1/uu/br_chapter-193513/pg-1/
While blondie is blowing mc we get those flashbacks. Of the picture of the brothers and the kiss with Hae-Yool.
Again mc is seeing the dead brother and Hae-Yool as objects of blondie's affections. His rivals.
Look at the ghost in the bed whispering to mc. Again, the dead brother in his school clothes.
And the flashback? It looks after they started to distance each other. Or is this imagined? Because from blondie's pov they never really split. Mc started to distance, the trip happened, and mc disappeared.
Ch 17
https://www.mangago.zone/read-manga/backlight_1/uu/br_chapter-200704/pg-1/
Reminder of the spiral after mc ran away and disappeared from blondie's life.
Ch 20
https://www.mangago.zone/read-manga/backlight_1/uu/br_chapter-205762/pg-1/
First look at how the misunderstandings started. Now look at the image of the brunette being kissed. Can't see the mole but it looks like the dead twin brother, right?
Ch 21
https://www.mangago.zone/read-manga/backlight_1/uu/br_chapter-206843/pg-1/
Another image of the kiss.
Ch 25
https://www.mangago.zone/read-manga/backlight_1/uu/br_chapter-213363/pg-1/
Blondie trying to find mc and the confusing heart break he is left with.
This guy who says that blondie has no right to look for mc is the other person I think the kiss happened with. At least he looks similar to person he kissed and this person's face is blurred and we never saw in mc's flashbacks.
I first thought him a friend of mc. Now on my third re-read, I think he might be the guy who got kissed only because it's weird how this person fits in with things.
This guy either had a crush on mc or blondie.
Ch 26
https://www.mangago.zone/read-manga/backlight_1/uu/br_chapter-216497/pg-1/
Mc and hyung's past.
That's why I am 98% sure the ones who kissed were brothers.
Because why else would mc see him as a rival and equate him to Hae-Yool?
What I understand of this story is this:
Blonde and mc were in love with other as kids but each thought it was unrequited but they were drifting towards each other and on the verge of confessing their love for each other.
Blondie's twin brother destroyed all that.
He set it up for mc to find blindie and him kissing. Mc thought all the practice was for the brother and started to distance himself.
Then their lives parted. Blondie had to leave on a family/modeling work trip of some sort.
Mc...I guess he was a bastard being raised by another man. His homelife was already violent and this made it worse to the point he ran away.
Blondie came back and everything he knew and loved was gone. Mc couldn't be found and no one will help him. He heard rumors and mc's friend, in part trying to protect mc from whom he thought was scum, warned him away.
Blondie was absolutely heartbroken and fell into deep despair and became a sex deviant. At some point, his brother, who destroyed his relationship with mc, died.
I also get the feeling that blondie hates his brother. Maybe the brother laid hands on him?
He certainly doesn't seem to know anything about how his brother ruined his relationship with mc and basically messed up everyone's lives.
The reason blondie is so violent with the men he sleeps with is because he is using them to vent all the love, hate, anger, rage, grief, trauma, jealousy, etc on them.
He needs a therapist. They all do.
Everyone involved in this twisted situation needs therapy.
Bottom line. Blondie and mc are STILL in love with each and still think it is unrequited. And they are both using each other in various means instead of COMMUNICATING about the past and how hurt they both were in thinking that they both were being used by the guy they loved in school.
Blondie and mc could be in a happy and healthy relationship with each other immediately except they are both hurt from having their love spurred by the other and blondie is way more messed up now than before and only knows how to use sex to vent.
They just need to TALK to each other. Just say, I was in love with you in school but I realized you just used me for sex and even 10 years later I still love you.
Then they can further talk and realize what happened to their relationship and why and start working on the now.
But instead we are going to have x amount of chapters of them hurting each other because they refuse to admit how hurt they were and how much they still love the other because unrequited love burns and destroys you in various ways because you have given up on ever having love and will settle for crumbs when the whole cake is within reach if only you reach for it.
Buckle up. We are in for a world of hurt with this story.