Yes, she does love Leon. In Resident Evil 2, she ended up falling in love with Leon (which is why she says she enjoys being with him). She saved him from the Tyrant, something that cleary went against her mission to obtain the G-virus. And though Leon was the one who kissed her, she would have done the same (and even does so in Darkside Chronicles). She also gives Leon a rocket launcher at the end. This is gone into further in RE4, where Ada can be hear saying Leon's name in a worried manner. She was very concerned to be seen with Leon when Wesker ordered her to kill Leon if she saw him. She even went out of her way to stop Krauser from killing Leon despite not being ordered to do so. She also assisted Leon by helping Leon kill Saddler and give him a rocket launcher (again). In RE6, she helped Leon (and Helena) with killing Deborah and Simmons as well as helping them in other ways. Despite all that I just said, I don't get where they stand. You would think that they would do something more than kiss on one occasion after knowing each other for 15 fucking years, but no they haven't. Ada is a very mysterious woman, something that might be the sole factor of why nothing else has happened between the two. I hope Capcom decides to make something of their relationship in the next game.