Honestly, I do hate Roy's character in Fire Emblem 6, but I admit there is a character. Roy can be pretty smart especially since he was able to peg the identity of Elphin. Roy can be pretty levelheaded in the face of dangerous situations and he's more down-to-earth than his dad. However, Roy is still 15 and his Smash fighting style is very flashy.
So, I think of Roy as a bit childish, relaxed, appears carefree, but is competent and knows when to be serious.
And I don't really consider Roy to be pure-hearted. That is not to say he isn't a good person (after all Ike isn't pure-hearted), but there's something about Roy that doesn't give me the same vibe as Eliwood, Lilina, and Mist (Tellius, but I think it's an appropriate comparison).