Vincent?

[Mercen-X]

He flew in Advent Children too, just taken as granted, in all fairness the whole universe doesn't really apply gravity

AC stands for Advent Children and therefore, it has already been mentioned . . . that means saying "too" is useless.

All the mako being sucked out of the planet lessens it's energy and thus it's mass and therefore there is less gravity than would be on our overweight planet

I like it. Although the creators themselves just do all this 'cause it looks cool. "Reason's got nothin' to do with it."

Vincent could jump in a very floaty manner in the game too. Like when the party first encounters him, and he emerges from his coffin.
Some side-effect of the physical and magical transformations wrought on him by Hojo's experiments, surely. Similar to Cloud's superhuman strength (and corresponding jumping ability), which can be attributed to Jenova cells, mako energy treatments, and such.

Vincent could fly because of Hojo's X-rated experimentations. Cloud, Tifa, Red, Cid, and Yuffie could all jump really high as well. Cloud is enhanced by Jenova cells and Mako overexposure. Tifa was trained by martial arts expert, Master Zangan and thusly defying gravity is no sweat.
Red is an animal and pretty much all animals defy human limitations. Given that he is a unique breed from a clan of self-proclaimed guardians of the planet, it's really no wonder he can leap tall buildings.
Cid, though a pilot by career is obviously a trained dragoon as evidenced by his spear attacks. Adding Highwind to his name isn't just a random decision but a reference to the Highwind who shared the same weapon choice in an earlier Fantasy. Regardless of whether he may or may not be a descendant of a dragoon Highwind, he possesses the superior abilities of a dragon diver nonetheless.
Yuffie is a ninja of Wutai . . . pretty much all of ninja in Japanese mythos are depicted having superior stealth and acrobatic capabilities which among other fantastic feats includes flipping gravity the bird.