Sotongie

Former footballer Hajun has been secretly in love with world-famous striker Mugyeom forever → Mugyeom barely remembers him → Hajun's career ends and he becomes a physical coach → Mugyeom returns to Korea and ends up at Hajun's club → Hajun desperately avoids him → Mugyeom's fuckboy radar detects CRUSH → Mugyeom starts teasing him → they start sleeping together casually → Hajun thinks he's living his dream while simultaneously emotionally killing himself → Mugyeom thinks this is casual while behaving like Hajun's possessive boyfriend → Hajun eventually realizes he cannot keep accepting sex instead of love → starts pulling away → Mugyeom discovers the horrifying truth that he has accidentally fallen completely in love → regret/chasing/growth → they properly get together → happy ending → FOUR VOLUMES of “Kim Mugyeom has a boyfriend now and everyone must deal with it.”

OHHH the romance is actually the real slow-burn mess underneath all the mutant apocalypse bullshit . And the important thing is: Cahaya and Seowoon do NOT suddenly fall in love during Naraka. They had feelings for each other all the way back in high school, and both spent years misunderstanding the other.
When they first meet in high school, Cahaya is the weirdly handsome transfer student who had lived in Indonesia, while Seowoon is this privileged-but-emotionally-repressed kid whose father hates Cahaya. Cahaya is poor and lives with his Indonesian uncle in a semi-basement; Seowoon comes from money. Despite the difference, they become extremely close. Cahaya follows Seowoon home because he's worried about him, messes with him constantly, defends him from classmates, and his uncle basically entrusts Cahaya to Seowoon. Seowoon already becomes emotionally attached to him without really knowing what to call it.
And here's the first big reveal later:
Seowoon had a crush on Cahaya in high school.
He just never confessed.
He secretly hoped Cahaya might like him too—until Cahaya got a girlfriend. Seowoon took that as confirmation that Cahaya only saw him as a close friend and buried the whole thing.
Meanwhile, Cahaya ALSO liked Seowoon.
For years.
So these two idiots were mutually interested in each other and somehow both concluded:
“Ah yes. Friendship.”

Things get worse after high school. They drift apart during university, partly because Seowoon's father strongly disapproves of Cahaya and partly because Seowoon is deliberately trying to move on from feelings he thinks are hopeless. At one point Cahaya sees Seowoon outside university with another person, gets jealous, and Seowoon deliberately ignores Cahaya when they cross paths.
Cahaya never forgets that.
Much later he actually asks Seowoon whether the girl he saw him with had been his girlfriend. Seowoon is basically like, what the fuck are you talking about, she was just a classmate, revealing that Cahaya had been jealous over absolutely nothing for years.
Then they reunite as Mansaengjong in Tacheon. They're adults now, but neither addresses the ancient emotional elephant in the room.
Cahaya dates women during the years they're apart.
And this becomes one of Seowoon's biggest sources of misunderstanding because he's thinking:
“Obviously Cahaya likes women. Obviously I was stupid for liking him.”
But Cahaya's relationships weren't especially deep. Later, when they're finally together, he tells Seowoon outright that he'd always liked him, including back in high school.
Then Naraka happens.
Seowoon loses his power, his guild turns on him, and he runs away. Cahaya later finds him again—and from this point, Cahaya's behavior toward him gets increasingly ridiculous.
He hunts him down.
He refuses to let him disappear again.
He gets irrational whenever someone threatens or insults Seowoon.
He remembers tiny details about what Seowoon likes.
He becomes insanely jealous.
And Seowoon still manages to think:
“Wow, what a devoted friend.”

The first major romantic break happens when Seowoon is asleep.
Cahaya kisses him.
Seowoon is actually awake enough to realize it happened, but instead of confronting Cahaya, he spends an absurd amount of time trying to rationalize it:
Maybe Cahaya was drunk.
Maybe he mistook Seowoon for an ex-girlfriend.
Maybe Seowoon dreamed it.
Anything except the obvious answer.
Meanwhile we get Cahaya's POV and discover that he is having a COMPLETE crisis.
He realizes he didn't kiss Seowoon as some joke. He's sexually attracted to him, got aroused from the kiss, and is horrified because he kissed Seowoon while Seowoon was sleeping. He literally compares himself to the creeps he despises.
And this is basically when Cahaya finally admits to himself:
Oh. This isn't friendship. I am fucking in love with this man.
Seowoon, meanwhile, has his own problem.
He already knows he likes Cahaya.
At one point someone jokes about the two of them dating and Cahaya gets furious—not because he's homophobic, but because he's jealous about Seowoon potentially being matched with somebody else.
Seowoon completely misunderstands the reaction and thinks:
If Cahaya is this disgusted by people joking about us, what would he think if he knew I actually liked him?
He's genuinely hurt.
Then Seowoon says something like:
“I wouldn't mind if a guy liked me.”
And someone suggests introducing him to another powerful male Mansaengjong.
Cahaya's brain finally leaves the building.
He grabs Seowoon and kisses him in the middle of the fucking plane.
Not a cute delicate kiss either.
A full berserk Cahaya malfunction.
Seowoon slaps him, Cahaya goes back for more, Seowoon bites his lip until it bleeds, and their guildmates literally have to restrain Cahaya while Han Donhee screams at Seowoon to RUN.
Afterward Seowoon FINALLY connects the dots:
The sleeping kiss.
The jealousy.
The weird behavior.
The violent reaction to another man potentially dating him.
And thinks:
“…Could Cahaya actually like me?”
Then we discover something even better:
Seowoon's feelings were never new either.
He describes his own feelings for Cahaya as deep and longstanding, something he'd been suppressing for years because he was terrified of destroying their friendship.
So the romance isn't:
Cahaya falls first → Seowoon eventually reciprocates.
It's actually:
Seowoon liked Cahaya in high school.
Cahaya liked Seowoon in high school.
Neither confessed.
Cahaya dated women.
Seowoon assumed he had no chance.
They separated.
Both remained emotionally hung up on each other.
They reunited.
And it takes THE APOCALYPSE for them to finally communicate.

Eventually Cahaya's feelings become impossible to hide. Han Donhee outright asks:
“Do you like Cha Seowoon?”
And Cahaya can't deny it.
Han Donhee is basically stunned because Cahaya had only dated women before. Cahaya himself is annoyed that everyone somehow finds the idea of him liking Seowoon unbelievable.
But Cahaya is still scared to confess properly.
Why?
Because Seowoon disappearing from his life once already traumatized him.
Cahaya thinks that if Seowoon rejects him and distances himself again, he would rather remain Seowoon's friend than lose him completely. His internal monologue is basically screaming:
“Can't you fucking SEE that I like you?”
Eventually Seowoon overhears enough to understand Cahaya really does have feelings for him.
After that, their dynamic changes dramatically.
They start having way more physical intimacy—hugging, lying together, Cahaya clinging to him, Seowoon slowly allowing things he would've previously panicked over.
And the funny part is Seowoon isn't actually resistant because he doesn't want Cahaya.
He's resistant because he's inexperienced as hell.
He tells Cahaya basically:
You're used to physical intimacy. I'm not.
Seowoon doesn't even hug his parents. Meanwhile Cahaya has dated before, so Seowoon feels completely out of his depth.
Then comes the moment where the misunderstanding finally dies.
Cahaya jokingly says that if they're going to spend their whole lives together, kissing shouldn't matter.
Seowoon fires back:
“Just say you want to kiss me.”
And then CLOSES HIS EYES.
Cahaya practically short-circuits.
This time Cahaya actually hesitates because he wants to know Seowoon is allowing it. He kisses him gently, Seowoon doesn't push him away, and when Cahaya asks whether he'd let other men do this too, Seowoon tells him:
Only you.
And THEN Cahaya realizes:
Oh fuck, I messed up the order.
Because this idiot has already:
kissed him while sleeping → gone feral on an airplane → basically dry-humped him → THEN remembers maybe he should actually ASK HIM OUT.

So Cahaya finally does it properly:
“I'll treat you really well. So, go out with me.”
And from there they're effectively a couple.
The really sweet later reveal is that Cahaya tells Seowoon:
he has always liked him.
Seowoon initially still misunderstands and thinks Cahaya means “as a friend.”
No bitch.
Cahaya means always always. Since high school.
And Cahaya also admits that he considers himself a true Mansaengjong — a late bloomer because although he loved Seowoon more than anyone else, he realized and acted on it ridiculously late.
Then the cosmic adventure turns their romance into something much bigger.
Their relationship basically becomes the emotional engine behind everything they do.
Cahaya's goal repeatedly becomes:
Seowoon must survive.
Seowoon's becomes:
A world where Cahaya doesn't exist isn't worth saving for me.
That's why the finale hits harder romantically than adventurously.
When Seowoon has to leave Cahaya behind in Aso, he finally tells him explicitly that he has loved him for a very long time and will continue loving him forever. Cahaya is crying and begging him not to go, but ultimately lets him because—as always—Seowoon's survival matters more to him than possessing Seowoon.
Then they are separated across fucking cosmic time.
Cahaya spends an unimaginable amount of time destroying Gates across the universe.
Seowoon waits on Earth.
And Cahaya eventually comes back because he'd promised Seowoon that even if they were separated by hundreds of billions of light-years, he would cross time and space to find him.
When they reunite, Seowoon breaks down because he'd genuinely believed he might never see Cahaya again.
Cahaya comes back.
New Year's bells ring.
And the final romantic image is basically:
They spent years failing to say “I like you,” then literally crossed planets, destroyed civilizations, death, reincarnation-ish cosmic states, and fucking spacetime itself just to get back to each other.
And THEN the side stories are almost offensively domestic.
They're back on a normal Earth, attending university together. Cahaya becomes a student largely because he wants to be at the same university as Seowoon. They're attached at the hip, talking about careers and starting a business.
Seowoon asks Cahaya what he wants to do for a living.
Cahaya's answer?
“Being Cha Seowoon's hubby.”