a sneaky rogue? a flirt? selfish? while ferio is or has been all of these things to an extent (moreso in the anime), i would never list them as ANY of his characteristics (well, maybe a flirt). when we first meet ferio in the anime, we learn that he's lived much of his life "on the streets," so to speak. when you grow up that way, the main rule to get by is to look out for number one. ferio doesn't have any reason to be unselfish; he has no people close to him.
in the anime, ferio has no memory of his early childhood. the first things he remembers is being trained by his master. in the manga, ferio still remembered that he was the princess' brother, but he had left her side for reasons unknown. his title of prince is a burden. as we quickly learn, he can take care of himself just fine.
while at first ferio is selfish and manipulative (he has a talent for using people and situations to his advantage), as the first season progresses, we see him change. by the time the second season rolls around and he is reunited with the magic knights, he is, on the surface, a completely different person. he is much quieter, less rowdy and flirty; he's been saddened by the death of his sister and sombered by the crmbling of his country.
we see ferio as being a very comforting and supportive friend. he grows up so much as the series goes on (moreso in the first season). it's very difficult to sum ferio up, because he's such a complex and realistic character. actually, fuu just barely beats him out as my favorite. ;D