Tifa was intended to be featured with Cloud and Sephiroth in the original Kingdom Hearts, but was removed due to time constraints.
Tifa, along with Cloud, Aerith, Yuffie, Cid, and Leon were originally set to appear in Kingdom Hearts Birth by Sleep. However, in the final release, they were removed from the game because, according to Nomura, there were too many plot lines already, and it would have gotten confusing.
Kingdom Hearts II
When Sora revisits Hollow Bastion for the first time, he meets Tifa, who inspects Ansem the Wise's study, looking for "a guy with spiky hair." Sora pulls on one of his own spikes before Tifa laughs and says "Spikier".
Tifa is actually looking for Cloud, and she nearly catches up to him during the Heartless invasion, but he chases after Sephiroth, leaving her to fight a large group of Heartless on her own. She later fights alongside Sora in the Ravine Trail, preceded by Yuffie and Leon, and followed by Cloud.
After Sora defeats Sephiroth in battle, he finds Cloud and tells him that Sephiroth is waiting for their battle. It is when this confrontation reaches its climax that Tifa finally catches up to Cloud. She tries to help Cloud, saying that all that needs to be done is to surround him in light, and telling Sephiroth that the darkness will always be there, but in a place that he can't reach. She fights with Sephiroth very briefly, then gives Cloud the light to overcome his darkness, after which the two men return to battle and disappear once again, leaving her to continue the pursuit. Before she does, however, Tifa gives Sora the Fenrir Keychain, thanking him for his help.
Tifa is portrayed as kind and strong, but reserved. She is shown to be unflinchingly loyal and persevering, as demonstrated by her continued quest to help Cloud, despite setbacks. She is brave and confident of her abilities, as shown by her brief fight against Sephiroth.
Like Cloud, Tifa's appearance in Kingdom Hearts II is based on her look in Final Fantasy VII: Advent Children. She has long, roughly waist-length, brown hair that is tucked behind her ear on the left side, showing that she has a silver, teardrop-shaped earring. Her eyes are rust colored and she seems to wear pink lipstick. She wears a black, leather tank top with a zipper down the front overtop a slightly longer, white tank top (similar to her original shirt in Final Fantasy VII). Tifa also wears a black, leather skort with two black ribbons tied on either leg. The back of this skort is quite long and reaches the floor. She also wears black, leather gloves and black and white shoes, with black ribbons tied around each.
Tifa possesses the same tremendous strength she did in Final Fantasy VII, easily capable of high level martial arts such as lifting a gigantic portrait of Xehanort, and kicking a wall hard enough to shake a whole room (her high physical strength being displayed in a comical way is a small running gag).
She can perform her final Limit Break attack called "Final Heaven", which enables her to send a massive energy blast at her opponent.
Tifa grew up in the same village as Cloud Strife, Nibelheim, until he left for Midgar to become a Shinra Soldier, not achieving SOLDIER yet. A few years later the fateful Nibelheim Incident took place. Sephiroth killed several people and burned down the village. Tifa, with Cloud and Zack, tried to fight Sephiroth. Tifa and Zack got beaten badly, but Cloud managed to defeat Sephiroth.
In the intervening years between this event and the beginning of Final Fantasy VII, Tifa trained in martial arts to become stronger and joined AVALANCHE, an anti-Shinra group, where she also finds Cloud. When they learn that Sephiroth is still alive and plotting evil, they decide to fight him again, with the help of other characters. This quest to find and battle Sephiroth makes up the bulk of Final Fantasy VII.