No one in RE canon loves Leon the way Chris does
The thing about Leon is, he has a lot of people who like him. He's a well-liked guy with very few actual personal enemies. But very few people tend to think about him beyond that point.
Claire likes Leon just fine, for example. So does Jill and Rebecca and Helena and Hunnigan and every other person who's ever worked with him. People value his insight and experience, they enjoy his company, and they overall have very high opinions of him -- but that's about it. None of them ever seem to go out of their way for him (except for Hunnigan, who only does it because it's her ass on the line if she doesn't) or have their lives affected by him in any personally meaningful way.
There are other people, like Ada, who have a certain degree of affection for him. Leon has a soft, special place in Ada's heart, sure -- but she also has a very out of sight, out of mind attitude towards him. The biggest debate in this fandom, sometimes, is what her actual feelings are.
And then there's someone like Ashley, who is the tragedy of the love that never was.
But then there's Chris.
Chris is the only person in this series who actually thinks about Leon just because he cares about him. There's actually a moment in RE6 where Chris radios him just to check on him and his location and make sure that he's not somewhere that's about to blow up or get destroyed.
The only time that was ever done in this series was in RE0 and RE1, because that was an entire team of people who all worked together trying to make it out together. After that point, everyone works pretty independently of each other, and any "check-ins" that happen end up happening purely by happenstance (think Leon and/or Claire actually glimpsing each other on a CCTV and trying to call out to the other at the end of RE2). But in RE6, Chris buzzes Leon out of nowhere just to check on him.
There's also a thing about Chris Redfield where -- one of his major character flaws is that he gets extreme tunnel vision. He's also stubborn as all hell and can't really be talked out of something once he sets his mind to it. Name a character that has teamed up with him at some point, and there's an example of him not listening to their plea/advice to do or not do something.
Chris listens to Leon every single time. Without question.
In RE6, Chris won't even listen to Piers when it comes to his blind fury and relentless pursuit of Ada -- Piers, who Chris handpicked himself to be his own successor.
Chris listens to Leon the first time, without question or much pushback, and he never goes back on it. All it takes is Leon saying one time "let the Ada thing go, I can speak on her behalf" -- and Chris, somehow, magically and miraculously... does. There's a real argument to be made here, due to this moment, that Chris trusts Leon as much as or more than he trusts Jill, even.
Throughout Vendetta, Chris defers to Leon's judgment and goes out of his way to get Leon at his side, when a simple phone call could have sufficed. And when he finds Leon in the state he's in, suddenly his focus isn't on getting his professional opinion anymore -- suddenly, it's all about Leon's physical and mental state. Chris is pissed not because Leon's refusing to help them, but because of Leon letting his depression win out and his drinking problem take over. Chris tries to take the booze from him not once but twice.
If Rebecca hadn't been there to keep them both on track, this would've stayed and continued a personal dispute. It wasn't about the mission anymore, for Chris. It was about Leon. And Rebecca had to remind him why they were even trying to talk to Leon in the first place.
And I know that there were Chreon moments in DI, but I've only watched it once so far, so I'd have to watch it again to talk further about it.
But suffice to say, Chris loves this man. Chris loves Leon more than Leon loves himself. That's for damn sure.
Hunnigan even describes them as a sort of love at first sight thing. It's impossible to know just how close they actually were in the time between their first conversation during the events of Code Veronica and their first face-to-face meeting in roughly 2008ish, but once they were together, they have had a partnership to rival any other in this series.
I actually hate to think how much worse Leon's depression might have gotten if not for Chris looking out for him -- because I think it's a really safe bet to say that they keep in touch between missions.
In fact, if the entire dev staff of Division 1 showed up at my house tomorrow and tried to say that Chris and Leon have no relationship outside of work, I'd call every single one of those devs liars LMAO