(Thanks so much!! It's always super reassuring to get compliments like that, lol.)
Okay, so you're already correct that his area would be misinformation, at least at first. (Incidentally, that is also true for Kyoko in the Despair's Throbbing Heart universe.) Towards the beginning of the despairening, Shuichi's role was to expose a strategic combination of real secrets and fake ones. So like, a high-profile murder happens, and Shuichi solves it without even being asked, revealing that the murderer was also very high-profile (ideally a politician). He nudges private scandals into the public eye. Private projects. And based on the legitimacy he gains from doing those things, he can suddenly say pretty much whatever divisive lies he wants, and people will believe him en masse before anyone with the mind to question it even gets a hand on the ball.
(Since this AU takes place after the V3 class have been Class 79, this suggests something must have gone down between Shuichi and Kyoko. After all, my Class 79 arrangement has him as Ultimate Detective's Assistant, and the Ultimate Detective's Assistant works with the Ultimate Detective, at least in theory. One assumes Class 78 is despair in this AU, since the initial ask about the V3 cast as despair was a follow-up to a Both Classes Despair AU post, so maybe Despair Kyoko and Despair Shuichi were confusing the masses by being at odds. They've both gained legitimacy by being frequently right, but then suddenly they start disagreeing. Very publicly and loudly. Sometimes Shuichi is lying and Kyoko isn't, sometimes it's the opposite, sometimes they're both lying, and sometimes they're both telling the truth in different ways. It makes people want to take a side, but picking either one will mean being misled most of the time.)
This role for them is only useful before the world powers collapse. Once everyone is kind of languishing in rubble anyway, there's no real point to spreading misinformation, or at least not as carefully as they were doing it. Flyers and wall graffiti are the main medium for lies at that point, and that's kind of not their area.
At that point, they're both more like spies, using their investigative skills to gain intel on resistance efforts.
Personality-wise, he's kind of the inverse of Despair Kokichi, in that Despair Kokichi presents as really high-energy in front of people and then is actually really somber and dull in private. Despair Shuichi does everything to come across as mopey and ignorable, but when by himself or among people who know him personally (his classmates, or Makoto), he reveals himself to be having a lot of fun.
For some length of time, he definitely toured with Kaede while she played music for people, because being her unassuming tagalong gave him access to large gatherings, where he could sneak around and overhear secrets and just generally notice things about people. Kaede plays one of her post-apocalypse concerts, and Shuichi slips away into the crowd and then returns to her afterward like, "There's a group of 'rebuilders' working out of the sewers. They've been storing up weapons down there, planning take the territory back from Owada in a few months."
"Sounds flammable," Kaede muses, and Shuichi grins.
(Unrelated, but Kaede loves piano so much, she definitely has to play wrong notes in order to get despair out of it. Like, if she played a song correctly and to completion, it would fill her with hope, so she has to make sure to play discordant notes periodically throughout, to remind herself that the idiots she's playing for don't notice or care, because nothing means anything.)
The relationship between Despair 79th Class and Despair 78th Class is more fond than the kind of frenemy relationship between the 78th and 77th, so Kaede and Sayaka might have done concerts together a few times. Assuming Makoto was with Sayaka during some or all of those times, he and Shuichi spent some time together, backstage.
"You look great," Shuichi says, his perceptive gaze sweeping over Makoto from head to toe and back. He still uses the bill of his cap to try to hide his looking, but he's not doing it now. "Looks like Maizono is...keeping you in comfort."
Makoto pulls self-consciously at the clothes Sayaka got for him. They're kind of gaudy and expensive, compared to what he's used to. "Uh, yeah. She is."
"Good," Shuichi laughs. "If she weren't, I'd ask Kaede to choke her out with her piano wire."
Makoto is unable to hide how his stomach drops, how his eyes widen. He heard, through other people, that their kouhais changed just like his classmates, just like their senpais, but...this is his first time meeting Shuichi after the biggest, most horrible, most tragic event. His first time hearing him make light of something so violent and laugh about it, with a carefree smile as if everything that happened, everything he's done, were nothing to him.
"Oh, s-sorry," Shuichi stammers, still smiling but now visibly nervous. "I forgot, you're still...so innocent."
A chill creeps up Makoto's spine, and he can't meet Shuichi's eyes anymore. It's not disgust; it's just so disheartening. He needs a minute to adjust. "I wouldn't call it innocence," he replies. "I just don't think any of this is right. Do you?"
Shuichi sidles closer. "Sometimes it takes an extreme calamity to give the world a chance to recover. Think about what would have happened if we'd allowed things to continue the way they were. All of the secret projects Kirigiri and I unearthed. Even thinking as small as Hope's Peak, they were never going to stop at just one Kamukura! They would have done it to as many people as they could get away with. We found a list, you know. A list of potential candidates, ones they were considering to be their second Kamukura..."
"I don't...know whether you're lying," Makoto says.
Shuichi lets out a laugh. A genuine one, and slightly grim. "I guess you don't. You weren't with us when we found it."
"The harm we do to people now isn't better than the harm they were doing to people then. Even if the school and the...the ones in power...The fact that they were hurting people doesn't mean we get to destroy everything. We're just hurting more of the same people who were getting hurt before!"
"Why do you always say 'we'?" Shuichi asks, tilting his head.
"If the point is to help people, then we should be helping them."
"I didn't mean to make you mad."
...Makoto sighs, at the sight of Shuichi's pouting face, his cap partially hiding his eyes. He places a hand on Shuichi's arm. "I know you're better than this. I know you. That's why..."
Shuichi lifts his head, but his eyes aren't meeting Makoto's eyes. They're on his neck instead. His fingers gingerly trail over the marks left by the collar Sayaka sometimes makes him wear (whether it's bruising or just a tan line). And he smiles. "You don't have to try so hard to hide that, you know. I-I think it's cute."
The concert starts, and Makoto slips away to give food and whatever else he can to those stranded out on the streets. And he does his best not to notice Shuichi tailing him the whole time.
