As I understand it's mainly based on sidequests. So if you do all of the sidequests with Tifa in Chapter 3, and none of Aerith's in Chapter...9, I think... then you'll get Tifa. And you have to do all of them in the relevant chapter for it to count. If you do all the sidequests for both of them then the tie-breaker is the choice in the sewers.
If you ignore all the sidequests and are a constant jerk to both of them you'll end up with Barret.
Chapter 8
Jack: How do you know?
Will: It's more of a feeling really.
Jack: Well, that's not scientific. Feeling isn't knowing. Feeling is believing. If you believe it, you can't know because there's no knowing what you believe. Then again, no one should believe what they know either. Once you know anything that anything becomes unbelievable if only by virtue of the fact you now... know it. You know?
Will: No.
If Demolition Man were remade today
Huxley: What's wrong? You broke contact.
Spartan: Contact? I didn't even touch you.
Huxley: Don't you want to make love?
Spartan: Is that what you call this? Why don't we just do it the old-fashioned way?
Huxley: NO!
Spartan: Whoa! Okay, calm down.
Huxley: Don't tell me to calm down!
Spartan: What's gotten into you? 'Cause it sure as hell wasn't me.
Huxley: Physical relations in the way of intercourse are no longer acceptable John Spartan.
Spartan: What? Why the hell not?
Huxley: It's the law, John. And for your information, the very idea that you suggested it makes me feel personally violated.
Spartan: Wait a minute... violated? Huxley what the hell are you accusing me of here?
Huxley: You need to leave, John.
Spartan: But Huxley.
Huxley: Get out!
Moments later Spartan is arrested for "violating" Huxley.
By the way, that's called satire. Get over it.
I kinda don't think it does.
Jack: How do you know?
Will: It's more of a feeling really.
Jack: Well, that's not scientific. Feeling isn't knowing. Feeling is believing. If you believe it, you can't know because there's no knowing what you believe. Then again, no one should believe what they know either. Once you know anything that anything becomes unbelievable if only by virtue of the fact you now... know it. You know?
Will: No.
If Demolition Man were remade today
Huxley: What's wrong? You broke contact.
Spartan: Contact? I didn't even touch you.
Huxley: Don't you want to make love?
Spartan: Is that what you call this? Why don't we just do it the old-fashioned way?
Huxley: NO!
Spartan: Whoa! Okay, calm down.
Huxley: Don't tell me to calm down!
Spartan: What's gotten into you? 'Cause it sure as hell wasn't me.
Huxley: Physical relations in the way of intercourse are no longer acceptable John Spartan.
Spartan: What? Why the hell not?
Huxley: It's the law, John. And for your information, the very idea that you suggested it makes me feel personally violated.
Spartan: Wait a minute... violated? Huxley what the hell are you accusing me of here?
Huxley: You need to leave, John.
Spartan: But Huxley.
Huxley: Get out!
Moments later Spartan is arrested for "violating" Huxley.
By the way, that's called satire. Get over it.