I LOVE Fai when he's funny, he seems so positive and he can drive Kurogane insane which makes me laugh, however, I CAN'T help but wonder what Fai's past was like, and I think everything get's so mysterious when Fai's serious side comes out, which I like, (I'm one of those people who like mystery's)
In other words, I like him either way, but I picked "Mysterious"