I have played KHBBS. But after 99% completion (damn that mysterious figure!) i grew bored... And if you remember, aqua carries ven into castle oblivion, ven touches with sora's heart, and sora's light is born along with keyblade powers! Terra touches riku, and gives him the power of the dark.
Terra is Xehanort.
Aqua gets thumbscrewed and rots in darkness, but she meets Diz, or ansem the wise. Then we await for KH3 so sora can save her!
So its not exactly "Happy" more like bittersweet. I like that in stories i guess.
But in most final fantasy games, you pull out in the end, win, and alls well that ends well so they are happy! :)