while i don't think any antagonists (besides izuru, who is more a manifestation of how fucked up the singular pursuit of talent is rather than a proper character anyway, and kazuo, who has literally no excuse) are bad, i definitely think monaka is the only one who manages to make the lightning of junko genuinely strike twice. you don't want to give junko too much nuance and tragic backstory because that muddies up her core motive; that she's an extremely intelligent analyst who's so understimulated that yanking on the world's strings until they broke was the only thing that made her feel anything.
but you can absolutely give that to another character, and choosing to give that to a kid that had been neglected her whole life both allows the kid to idolize the older character and give an organic build to that thing. monaka having a tragic backstory like the rest of the warriors of hope while simultaneously not really giving a shit about their children's paradise and really taking the shit junko was actually talking about on board makes for a fascinating character imo, and her emotional manipulation of everyone around her (weaponising kotoko's trauma and nagisa's crush to keep them in line is genuinely terrifying to watch, and it also leads to one of two genuinely well-made bad endings when she manages it with komaru)
and while im generally not in support of having characters feign disabilities, monaka is one of the few instances where it makes sense for multiple reasons, both in terms of her father and brother being shitty and working in conjunction with the emotional manipulator that monaka is, because it's very easy to sort of feel 'sorry' for a wheelchair using kid.