Yuffie has decided to associate many of the people who have influenced her life with a fruit. Why she has decided to do this is a mystery, but she has, and that's that. She will fulfill her mission at all costs- with the notable exception of the loss of her Materia. That will end the mission at once.
Vincent is a kiwi. To start with, he has a tough exterior (his leathers) and he's rather fuzzy. Inside, he's tart and full of little, sometimes unpleasant bits of his past/ Beyond those, there's a soft, sweet core that Yuffie is determined to how everyone.
Cloud is an orange. He has a tough, rather flashy skin that can take some work to get under. It covers a bunch of different pieces. A thin skin that serves more as a deterrent than an actual barrier covers each piece. Within the skin, he's tart and alive, even with the occasional seed.
Tifa is, without a doubt, a cherry. She's sweet, wonderful, fun, and perfect in everyway. Inside her soft core, however, is a solid core than makes her a formidable woman.
Cid is an apple. Sometimes a sour green one, sometimes a crab apple, and sometimes a perfect red one. He's firm, crisp, and to the point. There are no layers you have to peel back to get to him; he's right there and wonderfully reliable. Versatile too, and he never gets boring.
Sephiroth was a lime. Terribly sour, a real kick in the teeth without a pound of sugar, and not quite to the tastes of most people. He was admired for his exterior and his practical uses, but never for himself. Yuffie also doesn't want to see a slice of him in her drink.
Yuffie has never met Zack in the flesh, but she thinks he would have been a strawberry. Cloud described him as in-your-face fun, eager, enthusiastic, and always ready to cheer you up. He was spiky on top and had a bright, sunny attitude. No need for sugar with him.
Reno is a mango. He has a tough, leathery yet supple hide that takes punishment easily. Under that is an interestingly sweet and tart combination. Depending on the circumstances, he can be slippery, stringy, or sandpapery while still maintaining the sweetness. Like Tifa, he has a hard core that helps him weather the storms that come his way.
Tseng is a pineapple. Yuffie has mixed feelings about pineapples; they taste so good, but it's hard to get into them. Tseng, too, has a prickly, unfriendly outer coat that makes him hard to handle. His core is hard too, but not as hard as Reno's- he's able to bend when he has to. Between prickles and a hard place is a sweet, exotic region that one ought to linger over.
Finally, Rufus. He's a bunch of grapes. Green ones, with no seeds. A little work and time could turn him into champagne or a fine wine. Yuffie doesn't like wine.
Tifa would like to add that Yuffie is an apricot. Small, cute, sweet but with the potential to be painfully sour.