Love interest? Nah. Like BoB said, she's there to be Cloud's anchor to reality. She provides dramatic irony because she knows the reality of what happened but doesn't reveal it right away. In fact, in replaying the game recently, I'd say my favorite part was watching Tifa react to Cloud's inaccurate version of events. She's the replay value of the game, because you wouldn't realize on the first play that there's something truly flawed with Cloud's story, but in retrospect, her reactions are meant to reflect an incongruity in and an unease with the provided narrative.