Best Friends
Tifa didn't really have a best friend in her childhood, not even Cloud. He was just a good friend to her. Not a single person really cared to look deep enough or care enough about the young bartender after she moved to Midgar, not meeting many girls her age thanks to her profession.
One would suppose Marlene could have been Tifa's best friend, but no. The young brunette was too young, and too different to talk to. Aeris was probably Tifa's first best friend.
Aeris was gentle, with little physical strength to speak of, the opposite of Tifa. Tifa was loud and out-going, and a master of martial arts. But still, the girls had a lot in common. They were both motherly, and good cooks, and this soon bonded them, being the only girls among five girls. Yuffie, as well, was a welcome addition, although she didn't dig quite as deep into Tifa's heart as she had Aeris'. Aeris was like the mother Yuffie had never had.
After Aeris' death, the girls took it hard. Tifa was left to try and pick up the slack, cooking and cleaning. Yuffie tried to help, but she was usually distracted. It was in battle the two girls ended up bonding. Paired up with Red XIII, the girls became fast friends.
Tifa held back tears as Yuffie kissed her husband, happy that her best friend had found her man.