And now with that out of the way, here goes
Set somewhere after All Might's big fight against All For One (the one he has that is the reason he passes his quirk onto Deku in the main series), but before Deku meets All Might, being kind of what Better Call Saul is to Breaking Bad, a series that doesn't at all rely on its 'predecessor' to be good, and also gives backstories and development to several "ignored" MHA characters, including mainly how several UA teachers got in their line of work.
Main plot line is kinda just like if Deku hadn't been simped on by Ochako and hadn't gotten into UA, but still wanted to use his abilities to do good in the world, even if it meant becoming public enemy no. #1. It follows the life of Koichi Haimawari (a.k.a. The Crawler/The Hauler/The Cruller/Nice Guy), and him trying his best to make ends meet and do good deeds with his, frankly underwhelming, movement quirk (Slide and Glide, which allows the user to move in any direction at roughly the speed of a bicycle if they have 3 points of contact on the surface they're trying to propel off). One day on patrol, he witnesses Kazuho Haneyama (a.k.a. Pop☆Step, mainly just known as 'Pop') preforming an illegal idol concert out in the street, before the police comes and everyone runs away as to not get arrested. Later on, he finds Pop being mugged in an alleyway, and attempts to stop the muggers and save her, which, of course, goes terribly wrong, and while on the verge of being murdered, a grumpy old man, known as Knuckleduster (a.k- nvm thats a spoiler) who beats the shit out of the muggers. After this, Pop and Knuckleduster take refuge in Koichi's house (against his will) and their teaming up and story truly begins).
Like I mentioned previously, this manga also serves to give backstories, or just extended development, to previously neglected characters in the main series. This includes; giving Stain a backstory to why he became a villain; giving Oboro Shirakumo (the friend of Aizawa and Present Mic who is turned into Kurogiri, revealed in I believe season 5), Aizawa, Present Mic & Midnight an extensive explanation of their childhood, and explaining in detail what happened to Oboro; and finally a glimpse to Mirko as a schoolgirl, showing how badass she was even before the events of season 6
And one last thing, because of the whole gimmick of Pop being a pop(!) idol, there is a major subplot of idol culture, and every several arcs focusing on it, as to make it not just pure heroic action, but it is executed and flows with the story so well it shouldn't really take much away from you - best bet is to just read it :shrug:
Furuhashi kinda just took all the character dynamics from main MHA, and just removed all their flaws.

For starters, Koichi is kilometres better as a main character than Deku; he's not some 'chosen one' being carried in life by too much deus ex machina, he is just some guy trying to make his way through life, with development and power ups that have sensical placement (much better than One For All randomly releasing a new power stronger than the last one whenever Deku is in any minor peril - and as the story progresses, you realise every action this man makes is perfectly in characters, because he is just a kind ass motherfucker.

Next you have Pop, a very refreshing take on lead female characters in shounen, as she serves a purpose to the story past as being a milestone for the main character to work up so he can finally fuck; mostly shown through all the idol debacle, she could fully lead this series herself and I would be all here for it. Her relationship with Koichi isn't even something it makes much sense to heavily ship, they are just very close friends who can perfectly operate on their own separately, and honestly I'm happy this manga is underrated so people don't ship them on the level people do in main MHA.

Next we have Knuckleduster, kinda like the equivalent of All Might in Deku's life, but for Koichi. He is a lot more mysterious than the other two main characters, (which is why it's best you limit spoilers as much as possible because all the reveals are so expertly executed). He originally starts off just as this grumpy old guy who wants to the bare minimum, and while I really don't want to reveal much, you will slowly begin to realise how just wrong you were.
Now for side characters, which this series doesn't fail in either

First you have Christopher Skyline (a.k.a. Captain Celebrity), who is the Number 1 Pro Hero in America at this (which is confusing knowing about one of the arcs in the main series), who originally just seems like a phony doing all of this for clout (his name is literally celebrity for god's sake), but it quickly becomes clear that this is not the case and he instead because the actual insert for All Might for this series, trying in any way he can to assist Koichi and Pop, as long as he can get publicity from it.

Next you have his assistant, Makoto Tsukauchi, brother of Naomasa Tsukauchi from the mains series (the police detective that works with All Might, funny how that works), who is just always in a positive mood, and manages to light up any room or situation she finds herself in, when she's not busy pissing off her brother and getting in his way during cases. And to add, the majority of her facial expressions are reminiscent of Noelle from Deltarune, it is scarily uncanny.

Then of course we have Oboro Shirakumo who, while not entirely an original character, is giving more screen time than Horikoshi could ever dream of, so he fits here. The entire backstory arc showing his childhood, (alongside Aizawa, Present Mic & Midnight) was emotional the entire time, knowing he was not going to survive. He was a very fun and interesting character, with a really cool quirk idea - I really wish he could be expanded upon more.
Now for the villains, and while I will say they are not as good as Shigaraki (I really love Shigaraki, possibly the best part of MHA for me), they are in no way bad

The first one is a complicated case to explain, because it isn't just a single person, rather the villain itself is a quirk a parasitic 'queen bee' that inhabits peoples bodies, and turns them into villains (and of course all the people it inhabits are people important to the plot), which is honestly such an interesting concept, I can only hope Horikoshi would make a similar concept (just a rogue quirk without any actual owner).

And then you have Number 6 (a.k- nvm thats a spoiler), the kind of like overarching mysterious main villain, who gets even slower reveals than Knuckleduster, but honestly the tension as you slowly find more and more about him and follow him through his shenanigans is really great, and while I do prefer Shigaraki more, if this series was as long as main MHA, and he had gotten the same amount of time to develop, who knows what may have happened.

And then finally, there's pre-existing characters from main MHA who got more characterization within this. This series seriously made Midnight, Mirko and even one of Overhaul's random lackeys who got fuck all screen time in MHA some of my favourite characters in the series, because Furuhashi's ability to write and Betten Court's ability to draw exhillirated their every on screen moment ten fold
If you can't tell by now (if you actually bothered reading all that and didn't just skip to the conclusion, not that I would read this all either), I rank this spin-off much much higher than main My Hero Academia, and if even if you're someone who hates MHA, I will still wholly recommend you read this because of how it improves on everything MHA failed to do while adding more, and due to it being so underrated there isn't any fucking annoying teenage fujoshis shipping all the characters.
Honestly, if you like anything about superhero culture, or you're a fan of Love Live, this series should definitely be on your list.
This is the first time I've written a review (that isn't empty ramblings I posted to my account), and while this will probably become a rare happening for me (because I've realised how much worse my rambling and time management gets when it's all official-like), please feel free to criticise this or whatever, I don't really mind nor care (I just wanted to finally say I have written a proper review and what better to do it on than this)
88.5 out of 95 users liked this review