Going insane thinking about Grace being Leon's literal saving grace and how she forced him to drop the self-sacrificial lone wolf shit.
Leon, much like Grace, is very selfless. But the problem is that his selflessness comes in the form of self-sacrifice. Leon gets shot and is passing out? It's ok, he needs to make sure everyone else escapes first, even if that means leaving him to 'bleed out' alone. The government wants to murder Sherry? Yeah he'll gladly give up his own life to spare her. Leon's about to die from a parasite? He needs to make sure Ashley is ok first, even if that means he'll probably die. He doesn't live for himself, he doesn't value his own life beyond his use to others and his ability to save them, so if he has to sacrifice himself to save them, then at least he's served a purpose.
And with Grace, it's like looking in a haunted mirror. Young rookie cop/agent in their early 20s who joined the force because they wanted to make a difference only to have things spiral wildly out of control, get swept up in a bioterrorism incident in Raccoon City, meet a young girl along the way whom they then desperately try to protect, etc. The parallels* between Grace and 2R!Leon are actually insane like the game really does whack you over the head with it.
*and that was before we even got the full game; I still have to make a post about everything else we got in the full game
But despite the progress Leon made in DI, he's still very much the lone wolf. He's walking into what he knows is a suicide mission-- his body, his most important weapon is failing him, and he has no one to save him-- without backup. He's moody and broody and understandably upset, and he shoots down Sherry's attempts at checking on him, and she shoots down his attempts at isolating himself further. He's carrying around a fucking gun he named Requiem to honor the people he "failed" in Raccoon City and spends the game mourning the loss of people he didn't know or had only known for a short while. The RCI might've been 28 years ago, but Leon never left that city. He feels a part of him died there with the others, and thus he walks around like half a person, moving from one suicide missions to the next because at least if he gets killed in the pursuit of ending bioterrorism, his death will have meant something. He will have paid for his "failures" in RC with his own blood.
But Grace is persistent. She's determined and insightful and arguably maybe a little suicidal, but she's not going to let Leon die. And she throws the ridiculousness of the whole situation right in his face, without meaning to, and that seems to make Leon realize something. Yeah, Leon is sick as hell and dying and yet he still went on a suicide mission and that was kinda just a little bit stupid because he did not have to do this alone, he just feels like he did. Like it was his cross to bare. That he must suffer alone because that is what he's destined to do.
And she somewhat inadvertently forces him to reconcile with his own mortality because this is far from the first time Leon's almost died on a mission, or that his body has almost given in, but things are a bit different this time. There is no hope (that he knows of). And now it's not just Ashley or just Grace relying on him because there's a whole scheme going on behind the scenes too and if he fails, if he fucks this up, its going to be his worst nightmare-- Raccoon City all over again. And thus he tells her that if he dies, she must finish the mission. She has to promise to destroy the "virus," and Leon trusts her implicitly to do so.
But in the end, it's Grace's hope (literally!) that saves them. It's her persistence and her drive to not give up on anyone-- including Leon-- even when he became more of a "liability than an asset." He's now entirely reliant upon her for survival which is something he's never really had to grapple with before. Grace didn't really need him there to destroy the virus-- like Leon says, Zeno made it quite clear all she had to do was type in the wrong password-- but she saves him anyway. She refuses to let Leon continue with his death march and the second he goes to do his "I can buy you some time" (by sacrificing himself) routine, Grace outright tells him no. She has this handled, he just needs to sit tight.
Anyway, in the words of My Little Pony, friendship is magic and sometimes that magic is meeting the woman-version of your younger self who forces you to reconcile with your own humanity and prevents you from going on your suicide mission alone and makes you rely on others and that vulnerability, that "dependence" you seemed to always view as a weakness is what ultimately saves your ass. It amuses me greatly that every time Leon does the lone wolf act, the thing that ultimately pulls him through in the end is the person he either intended to save, or the friends he meets along the way, because even as strong and as capable as Leon is, he really can't do this alone.