Roxam has a real nice skill set, but I guess he is not as usefull in arena, because his AoE attack doesn't work until he has a (perfect) veil buff, so he can't use it in his first round. But in Dungeons he seems to be real great.
Harrier is probably one of the most useless champs in the game.
Knight Errant has, according to Ayumi, a 7 x atk damage multiplier on his A3. So he should hit really, really hard - but only single target and his def is a joke. 672 def at lvl 60, that is really not epic. A Swordsman common from the Bannerlords has more def than Knight Errant. You could use him in a very fast nuker team in Arena as 4th man, who takes out enemies that survived your AoE nuke or in a Cardinal-team.
Vrask is a solid healer and has a nice aura for dungeons.