Hoooly fuck take a shot every time I say 'like' here
I feel like that's natural actually. Catching a feelings can happen as fast as in a moment, and that moment was when yoojoon made jiho realize that his consent mattered and didn't just take advantage of him when he was in an unstable state. This moment was particularly touching to jiho probably because it probably felt like he wasn't in control his entire life, for example his dad seems to be a congressman and he's always instructed to be some type of way, along with the stuff about his mom, the way people might underestimate or overestimate him because of his beauty, and the way he acted out was probably to take back some sense of control. It makes total sense that he fell for yoojoon in the moment when yoojoon makes him realize that yoojuns adoration of him wasn't just shallow and he actually liked him as person with his own personality, thoughts, feelings and decisions. He made him feel like a human, and not some hollow doll to showcase or platform or take advantage of.
Like at the end of the day, Jiho was a character that was probably forced to grow up a little too fast, but I think the reason why jiho blushes and softens up around yoojoon is the authors way of saying that no matter how cynical or tough jiho presents himself to the world, he's actually still just a teenager growing up who is experiencing what seems to be his first love. Very sweet.
Yes, I get that, I mentioned that I like the effort to make it seem as realistic to how it might happen with an actual teenager. Jiho was aware of Yoojoon's sincerity before this but it was almost like a breaking point when he finally fell for him because of it. I just think it's so out of character and kind of messes with the flow of the story for him to be acting like that so suddenly. I feel like a complex character like Jiho would have a harder time sorting through those feelings of like. I think it's alright that Jiho developed a crush on Yoojoon, I just don't think it makes sense for those feelings to be so intense right off the bat and for him to come to terms with it so easily when all his life he's not known the concept of love. From a character like this I would expect his feelings to mature from interest to love as he gets to know his partner more, not infatuation.
I feel like even before this scene happened though Jiho already kinda liked Yoojoon, not even necessarily romantically before that, he's meant to be like an unreliable narrator of some sorts, like he was calling yoojun a psycho for being so obsessed with him, but according to his inner monologues he clearly kinda enjoyed his presence to a certain degree.
Also like I've mentioned before, Jiho is a complex character yes, but the author clearly wants to remind us that he is at the end of the day just a teenager who was forced to grow up in his circumstances. He's NOT mature and will not act mature. and when finally given the opportunity to have a safe space with yoojoon will act like the teenage boy he actually is because his edgy persona was developed to shield himself.
Feelings are also not necessarily logical, like imagine being a teen boy experiencing first love when you were so closed off, would you know how to act? I wonder if you've ever had a crush, it's difficult to act in front of them. Especially a sudden, intense first crush like Jiho's. Their actions can make you blush or stutter against your own volition, it's difficult to control and is so embarrassing because you're aware of how you act like normally.
He has no experience in love, from what we've read he's pretty much just had hookups. Again, he's fumbling around cuz he's a love noob and that might seem illogical to you cuz he doesn't act like an adult who has experience who in this case might've moved in the direction what you think Jiho should've done "according the his character setting".
In this case, it's also different because Yoojoon himself allows him to feel more confident in liking yoojoon back because yoojoon has clearly proven that he likes Jiho so much and he proved it in that scene when jiho tried to force himself on him. After that, even if it wasn't obvious, Jiho had begun to trust yoojoon with his feelings which is why he doesn't seem to be hiding his feelings as much as you think he should've According to his character setting. Your scenario might make more sense if Yoojoon has not shown himself likeing Jiho as much, leading to Jiho being more insecure. I'm not saying your scenario is impossible, but that the authors direction makes sense because they've already provided context for why jiho might "fall faster" as well.
It's not like Jiho suddenly became the caricature of a shojo mc, he clearly still retains sensibilities he held before like wrestling with the fact that yoojoon is bigger and stronger than him cuz he wants to retain his cool personality (again his desire to be in control) he became kind of a tsundere precisely because he's aware of how he acts in yoojoons presence and it kinda makes him mad because it's difficult for him to control it. I would assume he feels very vulnerable because he's experiencing falling in love for the first time.
The author actually does a very good job retaining his characterization and approaching his character evolution in a way that makes sense.
Like that's the point, it's supposed to be a bit illogical, it feels illogical, but the ingredients for that feeling to come to fruition actually already been sprinkled throughout the story.
Of course I do like the development I'm seeing in Jiho and how his feelings for Yujun are helping him get in touch with his other emotions, but I feel like the moment he fell for Yujun was too sudden? Like before that we didn't really get any indication of romantic feelings from his part (it was more like curiosity) and the fact that this happened like a single chapter after he admitted to being attracted to that girl's older brother. It was also kinda cringy how he was only there to "teach him a lesson" and then there's like no mention of his character after that. I def felt like him and his sister were going to play more major roles, but maybe we'll see in future chapters. I just feel like some Jiho could have some more time to sort his feelings through and get to know Yujun more, like become better friends with him, instead of instantly falling head over heels, nervous blushy mess. Like I know they're trying to show that he's just a realistic teenager but it just does not fit his character. Kind of just feels like author wanted to quickly get them together so we could move on to more major conflicts. Like maybe they felt like them being in love could help them get through those problems more easily, but I think them as friends and slowly falling in love while figuring things out would help their bond become even stronger. Idk, maybe it's just that I dislike fast paced stories.