I find it pretty interesting that Hinata is perceptive enough to somehow sense that Ko’s chilled, laid-back and easy-going attitude in the face of the exceedingly bizarre situation they were thrown into, by some ridiculous rabbit mascot, on their supposed first day at Hope’s Peak, was a result of some past trauma and, while that reading might seem entirely correct upon a first glimpse, I don’t think that was all there is to it, as this barely touches the surface and there is a lot more nuance to how he acts in the beginning, specifically in the prologue and the first chapter.
We do know from Ko’s Island Mode interactions, particularly one of the amusement park ones, that he is always afraid, that he is always anxious, giving him no compelling reason to enter a haunted house to feel fear, as this is just the norm for him and is the daily life he has been used to since long ago. With that being said, his response to Monomi, right after she informed everyone that the island they have been mysteriously transported to is an uninhabited one, was to frighteningly ask if she brought them there to forcibly make them kill each other. As far as I know, many people are dismissive of that line, thinking it is a little bit suspicious at first glance that he is seemingly vaguely aware of the premise, then they forget about it altogether and deem it irrelevant, undeserving of being remembered, when it is an important bit of characterization as well as an integral piece of the puzzle that reveals why exactly Ko was acting the way he was.
The way I see it, it is evidence that he was indeed afraid and he did find their absurd circumstances rather worrying, perhaps to a greater extent than anyone else. Ko is a fundamentally honest person and had absolutely no reason whatsoever to fake a reaction there, as it would not achieve anything and is not aimed at accomplishing any goal, so we can safely take that as an objective truth and his genuine initial reaction.
This is precisely where his first FTE comes in to re-contextualize his behavior throughout the prologue and first chapter and offer a new interpretation. When trying to help Hinata remember the talent he purportedly possesses but completely forgot about for unknown reasons, Ko assigns him with, before anything else, the “Ultimate Serenity” title and, when asked by Hinata about the part of him he looked at to come up with such a thing, Ko’s answer was that it is his ability to bring tranquility to others and that, in of itself, is an amazing talent brimming with hope.
Behind this veil of casualness, a hidden message begins to reveal itself in retrospect. Hinata’s sheer presence was enough for Ko to remain optimistic to such a degree and continuously reassure Hinata that everything will be alright in return, by calming him down when he is getting too panicky and providing some sensible explanation to any seemingly unrealistic events that kept occurring, such as Monomi turning a chicken into a moo-cow. The fact that Hinata was there, around him and by his side, just being who he is, helped curb his constant anxiety and aided him in not thinking too badly of the situation at hand. This was definitely not a one-sided sentiment, as Ko’s carefree and charming laugh was plenty enough to deflect Hinata’s worries and concerns. A truly befitting birth of an inseparable bond formed between two Ultimate Serenities.
There is another exchange that backs up everything I have explained above. The morning after the revelation of the first motive, we see that Ko was down, but a chit-chat with Hinata is simply enough to make him feel self-assured once again.
I definitely do think that Ko was the most affected by the first motive out of his peers (which is one of the many Sayaka parallels with the exception that Ko was more subtle about it and didn’t have an anxiety attack before Hinata), because it would not make sense to show his reaction to it before anyone else’s otherwise.
It is important to keep in mind that Ko is terminally ill and according to the diagnosis he received right before he got accepted into Hope’s Peak, he was told that he only has between half a year to a year left to live and knowing that he has lost his school memories complicated that a whole lot, as he no longer had any idea how much time he had left before he is a corpse. He wanted to influence the world for the better and he does not want to die a meaningless death. When you consider the situation he was in - stuck in a remote island without any means for help and no way to tell when he will drop dead, he felt compelled to do what he did, enacting a suicidal plan to manipulate Teruteru into murdering him so two hopes can clash with one another, which would either save one person or help make the rest of the ultimates stronger.
There were many factors at play here, but I think the lost memories motive was ultimately the trigger, or the straw that broke the camel’s back, if you will. I have already mentioned this earlier and I will repeat it one more time: Ko is an honest person who only lies when necessary. He only came up with his plan the night after the motive was given. It was an impulsive action. Ko did not endorse the killing game and was not planning anything before that point. This is also proven by what he told Fuyuhiko and everyone else in response to Fuyuhiko that he’s capable of killing someone.
I really, really love this about DR and FTEs. So much depth can be unlocked when you go back to certain scenes after going through FTEs and gaining a better understanding of the characters as a result. This just so happens to be one of my favorite examples that I wanted to talk about thoroughly, but there are definitely others, such as Twogami’s stance towards the killing game and Mahiru teaching Hiyoko how to tie her own kimono, which led to the formation of their bond. It simply resonates with me so much, and, as someone who struggles with anxiety issues, it was one of the first few reasons that made me fall in love with Komaeda (and by extension Komahina). DR2’s prologue holds the most value to me out of any of the main games’ prologues, it’s so endearing and re-visiting it is enough to make me feel at peace.