I love thinking about pre-JJK Yuuji because the two words/qualities that always ultimately end up coming to my mind are "aimless" and "bored" and I think that's very much worth thinking and writing about.
We don't know much about him before he enrolls into Jujutsu High. We get a few flashbacks here and there and as for the rest we're stuck having to infer from what Yuuji has to say about himself and this time of his life, but it is not very much, because despite how friendly Yuuji is, he actually quite rarely talks about himself and his past. Whether that's him making a conscious decision to be private or more so that he doesn't think to tell people about himself, I'm not entirely sure, though I'm more inclined towards the latter- specifically because of this :
He admits to trying to not be someone who overthinks too much, but you're free to disagree !
Anyway- aimless.
I look at Yuuji from pre-JJK, and I see a teenager who was already profoundly bored with life at his little age.
He was always very aware of his strength. He's the strongest person he knows (and by an extremely large margin) and you can tell he's confident about it. He goes in first when the occult club is visiting (allegedly) haunted places with no fear, he confidently (humiliates) challenges his sport teacher to keep his spot in said occult club, doesn't hesitate to take on a group of high school bullies as a middle schooler :
So he has that going for him, certainly. But that's not fulfilling for him. He's bored.
Yuuji could hang out with people, make friends. There are already people he likes fine enough, and people who like him fine enough in return, too, that they'd might even like to become closer friends with him. But he doesn't do proper friendships- he prefers spending his time reading shonens, hanging out with his grandpa, watching TV, going to pachinko parlors, and playing vigilante. See case in point : his classmates are gossiping about girls, and he's far away from them, and a whole lot more interested in his manga than in talking to them.
They're not interesting to him. Their conversations are boring. You can see his global disinterest manifest in his barely existent relationship with his middle school classmates, in the high school club he chooses to join solely because it ends at 5pm- It's like he has no passion, no fire (hello Hakari), no real interest in participating. He doesn't even care about having to leave his old life behind now that his grandpa is gone.
It actually makes me think about Sukuna and how everything he used to do in his life was just a way to pass the time. I believe that's similar to how Yuuji felt about his own life pre-JJK. Pachinko parlors, fighting, spending most of his free time reading shonens and watching TV shows- it's like he was just floating through the motions. Like regular life wasn't exciting or interesting enough. School, friends, girlfriends- he doesn't care for it. He's very much in his own little world. And, you'll either follow me in this or you won't- but I'm very willing to bet he used to fantasize about putting that unique strength of his to good use in much the same ways characters in the shonens he reads did. I believe he used to look at these characters with superhuman abilities and project onto them, want to lead a life similar to theirs. You can see here that when he starts training with Gojo and Yuuji asks him to help him become stronger so he can actually save people, his mind gets stuck on abilities he's seen superheroes and shonen characters have :
He gets soooo upset at the thought of not having proper "magic powers" that I'm so sure younger Yuuji used to fantasize in the privacy of his own mind about being like the overpowered MC of these mangas he was reading. Life is boring to him, and he's already halfway there anyway- he has the superhuman strength- but he doesn't have the other half : the opportunity to become one. A hero. He perks up immediately when Gojo tells him he doesn't need magic powers to become one, his physical abilities are enough. He can be a hero after all (not Gojo's words, just this particular Yuuji's at this point in time filtered version of it.)
(I also believe these shonen mangas are partially what informs early Yuuji's original opinion on what having "superpowers" or superhuman strength should be about. That if you have the power to be a positive force in the world, you should do it, point blank. It leaves him doing vigilante work as a teen, but also bored that there isn't ultimately more for him to do with that unique strength of his.
Megumi has to remind him TWICE that the point of being a Jujutsu sorcerer isn't to be a hero but to just try to do your best with what you're given. And in Jujutsu society, you're not given much.
But my original point will slowly get away from me if I elaborate more on the concept, or rather Yuuji's concept of heroism- so I'll get back on my original track right this second, lol.) I do not believe the urge to do good deeds/be a positive presence on earth only came to him once he joined Jujutsu society. That much is very clear to me from him beating up bullies often enough he earned himself a nickname for it (and not just for the hell of fighting, I believe it really was in an effort to defend 'the weak" for sure), or how often he visited his grandpa in the hospital, or those bouquets of flowers he kept getting for the nurses (and surely for grandpa too, a little bit) or how he enjoyed shoveling snow in the winter I believe partially so that his aging grandpa didn't have to, and all these little bits and pieces we know of younger Yuuji. He's good natured regardless of Jujutsu. But he was at a loss on what to do with that unique strength of his. He's not employing his "full potential" and I believe that ate at him a little bit, even unconsciously. That it's one of the reasons why he seems borderline apathetic about participating in Society with a capital S as a teen. It's not enough for him. He's bored. It's just mangas, vigilante work, tv, grandpa, repeat, for him.
Enrolling in jujutsu high gives him the one big thing he lacked previously : a way/opportunity to channel his strength into something bigger than that, bigger than himself, a way to channel his strength into fighting actual existential threats.
Curses and curse users are in an entirely different ballpark than fighting high school bullies is. The fire was there and ready to be fanned, the passion ready to be invigorated, he just needed something bigger than regular life could ever hope to offer him to make it properly come to life. It's so clear to me. He was bored and he needed a role- something only he could do that would put his unique strength to actual use like no good ol' "regular life" opportunities ever could do. Only then does life become intense and interesting enough for him. Only then there is a proper point to it all, to having this special superhuman strength. This is something only he can do.
I look at Yuuji pre-JJK and see a teenager who was aimless and very bored. Too powerful for his own good, confident and scared of nothing and no one. Few shits given for consequences. Pachinko parlors for the thrill. Not trying to make proper friends. Disinterested with the study hard, find a girlfriend, work hard, start a family and retire type of life. He just escapes his boredom through shonen mangas and TV shows, he does his good deeds through vigilante work and taking care of his grandpa, and repeats that indefinitely. He's bored and aimless.
Whether he was conscious or not of the nature of this boredom, you're once again free to choose, and I'm once again inclined towards the latter (re: as seen above, he tries to not to be an over-thinker + he's fifteen and fifteen year olds aren't exactly the most self-aware of age groups) but whether or not he was has no impact on my actual interpretation of his character pre-JJK.
Bored and aimless are the two main adjectives that come to my mind when I think of him, like he's always craved something more out of life but couldn't exactly dream of having it, before he was eventually pulled into the world of curses and sorcery, where everything is intense and... existential, by nature. The job, the daily life, the relationships, everything. Just like I think he unconsciously craved. (maybe you can blame Kenjaku and his tampering with Yuuji's genetics for it. :)
