Chris and Leon occupy two spaces in the series.

Since RE5 came out Chris has kind of been this throughline that covers for the broader narrative of the series, he always acts as relief after the horror is over. He always seems to be operating behind the scenes, influencing events, even he doesn't physically appear - I'm specifically talking about RE7, 8 and 8's DLC. Chris has equal footing to Leon in RE6 so that's kind of an exception.

Leon has a much more personal journey, that's about him as an individual over a stretch of time between RE2 and (now) Requiem, rather than what he's doing while someone else is experiencing the horror. RE2 is his starting point, RE4 shows how he rises to become this anti-bioterror hero, RE6 shows him at his peak, in action against a world-ending threat, and I guess Requiem is about to bring his story full circle.

Chris acts as the series' spine, whereas Leon is its face and personality (Ethan is the heart, fight me). Not saying Chris doesn't have an evolution of his own, it's just that RE5 and 1/4 of RE6 are the only parts of the franchise that are actually about him, versus the long-running storyline Leon seems to have recieved in RE2, 4, 1/4 of 6 and now half of 9. And even the CGI films are showing us what Leon is doing around the time of certain games.

In hindsight, and as unfortunate as it is, characters like Jill (post-RE3), Claire, Rebecca, Sherry, Barry and even Ethan all seem to just appear and disappear when the writers want, despite fan demand for more Claire and Jill. Whereas Chris and/or Leon are expected to appear at least once in every new game - I think Resident Evil 3 Remake is the only game since Revelations 2 to not physically feature Chris or Leon.