So what really happened to Korekiyo in chapter 4? Time to give a proper explanation for this! This is based on my Danganronpa V3 AU rewrite story obviously! TW: s*cide mentioned!
So we all remember from chapter 4, when the motive is a literal maze game. And in order to participate in, you must have a partner to fullfil the tasks. (Kaede with Rantaro, Shuichi with Kaito, Ryoma with Kokichi and Miu with Tenko) And Korekiyo ended up with nobody. He thought it wasn't a big deal and decided to enter the game anyway But oops, Monokuma said going alone is against the rules so he immediately had to be disqualified. What a shame, right?
So bc of that, Monokuma summoned the Exicals of two (red and orange) Monokubs, and immediately grabbed Kiyo to send him somewhere away to be watched. As he was sent to the classroom, watched by Monokubs (and guess what, they're still stupid). These two started to argue for no reason (either for other Monokubs deaths or who's more responsible) and Korekiyo basically used it as an opportunity to immediately locked them in the classroom and escape lmao.
Korekiyo thought it's better to hide in the dormitory, (to also rest for himself), kinda wonders how his friends are doing rn in the maze game.
When he entered the dormitory, however, he noticed something strange. Why was Tsumugi's room left open suddenly? He doesn't remember it being open when they left the dormitory? His curiosity made him look at what's left in the dead girl's room? A flashback light? Why's that item here? (He clearly remember the one which Miu fixed and placed in the kitchen. But perhaps, he has an opportunity to have more memories to remember, more than anyone in this academy.
But strangely, his "sister" told him something strange in his mind "Remember dear brother, this might bring many consequences". Kiyo actually got confused what she meant by that... But in the end.
What he didn't know is that this flashback light is different from the other one. Cause this one brought his REAL Pregame memories, everything before he joined the game. All of the memories of his previous life. When he tried it he didn't understand what the heck were these all, and he had to try it again,...until he eventually managed to remember almost "everything" . He finally remembered what was going on. All of this killing game, his sister usage for his character, even remember that time when he hnged himself. It all made him horribly sick, made him throw up...
In the bathroom of this room, he realized that "sister" inside of him dissapeared. The way he realized how they used his real dead sister for this story made him feel completely insulted. And pulled the mask from his neck to see the actual scars.
Everything was too much to the point he almost passed out. After him passing out, Monokubs came to check on him and realized what had happened and immediately panicked. (Had to sent him to medical room in the academy) And yes, bc Kiyo wasn't in the maze game and his state, he didn't participate in this trial 4.