I'm not double checking this right now, so this might not be accurate, but as far as I'm recalling right now Chris pretty consistently calls Leon Leon. I'm sure there's times in canon where he calls Leon by Kennedy, but honestly Leon is mostly referred to by everyone as Leon.
like yes he is also Agent Kennedy, but I feel like characters drop that so fast. and I'm pretty sure that in some material he's straight up told people to just call him Leon, which makes a lot of sense. I don't think Leon would love to be referred to as Agent Kennedy when his job is barely his choice.
however, Leon seems to change what he calls Chris purely by his mood. like in RE6 I believe he was very consistent in calling him Chris, but in Vendetta that man was Redfield.
again I am not fact checking this so do not take this as gospel, I'm honestly just rambling about these two. and Leon's relationships with names is a fun thing to dive into.








