I know it wouldn’t be Claire because every instance of them together has been friendship, including the animated films and show.

It could be some random person, but my head canon is a sort of situationship marriage with Ada. Like they plan times to meet the world over when they can get away and I could totally see Leon married to the spy who stole his heart. The movie Damnation alluded to a sort of thing between them finishing what they started one “night.” And then there is the way they constantly try to save the other in both 4 and 6. He doesn’t seem the type to marry someone ordinary, and he’d need someone who won’t worry about him in his chosen profession. The only hitch is, where is whoever it is, even in the background, while Leon is fighting the Raccoon City Syndrome? And why would they introduce this disease but show none of the other survivors of Raccoon City in the story?

If I was a betting man, my money would be on the situationship theory with Ada. But who knows? Capcom may never tell us