When I picked this webtoon the first time, I have only read the first season and I didn't have any desire for more. I was more enamored with your usual power fantasy transmigration/reincarnation opmcs, and Min is definitely not an op protagonist, even in later seasons (altough doing parkour gives him mad skills). The second time - I have to admit, it's one of the best action/thriller webtoons out there, really thought through.
But! The problem also lies in the title - Parallel City gives a huge spoiler, that the doppelgangers are not clones, aliens or whatever, but people from different dimensions. And the first season is more slow burn mystery than an action flick - there is considerable build up to reveals, and it's better to read it all at once than in episodes. The format of the webtoon releasing weekly episodes didn't do this title justice.
But maaaaan, when the action scenes hit, they are superb. All the parkour is greatly done, we see all the moves very clearly and it's pretty realistic. There is a great twist at the end of season one (not spoilering this one), and the action and tension picks up in seasons two and three, right through the emotional climax.
I did mention earlier that we don't have opmc - Min is just normal teenager with love of parkour, many friends and no direction in life. He will go through hell and slowly grow up, but with him it's more like he becomes sharper and more focused, rather than changed completely. I like that there are no characters that feel like cardboard, all of them could tell you their own stories, all of them feel like someone you could meet. There are stakes attached to characters actions, the plot is consistent and just rifled with tension.
I could go on and on about this webtoon, since I have found a lot of fascinating stuff and characters here. But it's too long already, so I will end this saying that it is a great story, although not your usual escapis/fanficky one, so popular right now with webtoons. But give it a chance, especially now when it's completed.