Love interest Makima, Death, Yoru aka War(Asa aka Host) and Quanxi(her Fiends count with her unless you don't want to do them) and finally Reze
Our Mc will be the same age as Denji after all our Mc takes his place. Nice? healthy? mentally sound? Humanized? Makima take your pick either way Makima genuinely cares about our Mc gradually aka Makima gets her wish dont really know how to put it.
Mc is a hero or anti Hero aka Underground Hero story headcannon underground hero's in the Mha universe are all or mostly Anti Hero's that's why they don't like to be known though this is unknown the the civilians.
So this can work in two ways our Mc can summon those from the Chainsaw man universe or they can be combined in to one world with the plots happening simultaneously or one after the other UA to Chainsaw Man or other way around. If Mc summons she starts with one Four horsemen Makima(control) Death War and Famine or the chainsaw devil (Pochita) other devils can happen naturally like in the Chainsaw man universe this only applies for after she summons all four horsemen or she can summon them though they will be weak at the start and will have to accumulate fear this doesn't apply to the 5 mentioned before. The Devils she summons aren't controlled by Mc while they are semi loyal to her and unable to harm her they still have their instincts as Devils or if they are human like Quanxi or Aki they still are themselves just protective of Mc this does not apply to those around Mc only to the Mc herself. The Mc does not have control of who she summons even if she summoned Makima first she still wouldn't be able to control it. with this version once she summons a Devil the two universes will start to combine world rules in other words the Devil's will still be bound by their world rules and Mha people could develop devil related Quirks though the core devils will always be just that Devils unless they become Fiends or pull a Denji and Pochita or in this case Mc and Pochita. For the second way this works Makima either finds Mc when she is still a child or her parents make a contract with Makima before they died and Makima finds our Mc after she has taken the UA entrance exam either way Mc is both a Devil hunter and a hero student how that happens you get to decide.