The difference between Arthur and Lancelot is that Arthur has to be hit over the head with something before he sees it. Lancelot sees everything. We'll there is also the fact that Merlin acts like an idiot around Arthur to keep it a secret.
Merlin + Harry Potter Tough I don't see Merlin as a Slytherin at least not if he was to go now. I think he would be Ravenclaw and he is in my story. <<< The Great Merlin was a Slytherin hence the image would be correct
Arthur is in the heat of battle and all gun-ho and then he turns and sees Merlin and he just turns into a little kid. He looks so devastated and just stares for a moment like he isn't even comprehending what he is seeing.
"These line always make me sad because Mordred was one of the few who still saw a hope for Morgana." Mordred saw the good in her when no one else did. There was still innocence left. The others were (understandably) blind to it, but not Mordred.