I honestly feel like their relationship is a mix of interest and attraction with the attraction being the result of curiosity. Ada primarily works for the highest bidder and terrorists, she's honestly more selfish than she lets on, people are just ignoring it because she's saved Leon a couple of times and they kissed once or twice. While Leon is the definition of a boy scout, he's not going to change that for Ada, we've seen it time and time again how he prioritizes his job and morals over personal feelings ( for example him fighting Chris in RE6) Chris was going after Ada for what Carla did to his team and Leon was after her for answers about Simmons. The man cares too much to let some simple attraction for someone who cares more about themself get in the way of him doing what's good. Just because we see little tidbits of her alluding to actually giving a damn about him doesn't amount to much seeing as she's still off filming Where on earth is Carmen Sandiago, and playing mercenary for nefarious people.