1) Reason for not using the Raijin
Anime
After saving Fuuko and Domon from drowing, he tells Fuuko that the reason why he will not use the Raijin is because the Raijin and the Fuujin are destined to fight one another.
Manga
During his battle with Noroi, Raiha let himself get beaten up. When Kurei asked him why he did not use the Raijin, he told him that it was because he would only use it if and when Kurei was ever in danger and needed help.
2) His relationship with Fuuko
Anime
It's more hinted at than shown, really, since he's given extremely little air time.
Manga
He DEFINITELY has a liking for Fuuko. So far, from manga 6-13, he meets up with Fuuko about 3 or 4 times, and he gets, how shall I say it, friendlier and friendlier each time. ^_^ That's why you'll notice almost everything in my "Proof of Love" page is from the manga.
3) His behaviour
Anime
He appears serious and solemn.
Manga
Here he is portrayed as really goofy, even tripping as he walks up the steps leading to the stage! (He looks so kawaii! ^_^) He also has lots of admirers, mostly female. ^_^