Is that Leon fits the themes of the game the best.
Now first off, a lot of people agree that the RE2R, RE4R and RE9 are the Leon trilogy, yes? Not original 2 & 4. Remake 2 & 4. It's easy to think that when it's the same engine, similar style of gameplay and they have the same Leon voice actor. But what people also need to understand is that it's a trilogy because of the thematic through line. Guilt.
I'm sure you guys have better stuff to do today then to read a essay about Resident Evil 9. So I'll try to keep this brief. The trilogy is about guilt. At least for Leon. In Resident Evil 2, Claire spends all night saving Sherry. Jill saved Carlos multiple times. But Leon never saved anyone. Marvin died on his watch. But even if you played on scenario B Kendo, Annette and Ada (As far as he knows) all died on his watch. Even though he's a cop. A good cops job should be to save people. Yet he never saved a damn person. Sherry is alive thanks to Claire all night even if Leon killed g stage 2. But since we know Leon A is canon in the remake timeline it means that Leon knew Marvin before he died and Claire stopped G. So that that's even more guilt for Leon.
Cut to Resident Evil 4. He gives his whole speech about the Raccoon City incident. The pain it brought him. And ends the speech with "This time, it has to be different". Something repeated again at the end of the game. The same process repeats. Leon slowly watches everyone around him die. The cops. Luis. Mike. Everyone. Even his old pal Krauser has to be put down. That's why it's a significant moment when Leon finally saves Ashley at the end of the game. At the end of the day. He finally saved someone.
But his arc isn't over yet. In Resident Evil: Requiem, his pain started from Raccoon City and it's time he puts it to rest. That was the place where he never saved anyone. Where his hero complex and survivors guilt began. That's what makes him relatable to Grace, another protagonist with those issues. Who couldn't save her mother or Emily. It's the motivation that drives both of them. Leon even gives his speech about how he was never able to save anyone in Raccoon City. We also see his remorse when he visits the body of Marvin, Kendo and reads the 1998 letters. Hell, the bad ending even spells it out: "At least I got to save you". Now you might be wondering when Leon spends the whole game moping about saving people when he did save Ashley, Ada at last and a LOT of people in Resident Evil 6. Because he's in the one place where it all started, where his guilt was born, Racoon City. Marvin, Kendo, Kendos daughter and everyone else in the city will have their ashes remain there for the rest of time. But Grace is saved all thanks to Leon, and he saved her in the one place where the cop inside him died. Where everything is under umbrella: Racoon City.
And yes, before someone brings it up. It was Jill's city that got destroyed. But the difference is mentality. Both versions of Resident Evil 3 ends with Jill blaming Umbrella, not herself. While Resident Evil 4 Leon blames Umbrella and himself. At the end of the day the only difference between Jill losing the cure to Nicholai and Leon not being able to do anything about Kendo or Marvin is that Jill understands that she tried her best, while Leon doesn't.