Yami asks William to take of his mask - William mentions not many people have seen his face; and also brings up the fact he knows why Yami is suspicious of him. Yami thinks of the reason why he is sus; his physique, voice and presence. Yami mentions their past together as magic knights and William eventually takes off his mask. (As you can see in an image above)
The scar was from birth (not too sure about that...) cursed child? He was fears by 'humans', a noble welcomed him into their family but the wife abused him daily. He met Julius who did not care about his appearance but was amazed by his magic and offers him a spot in the magic knights, but William is hesistant because of his face; then Julius gives him the mask he wears till this day.
Some more stuff it mentioned between Yami and William, but Yami apologises and William thanks him.
Something like this ^ But, I'd still wait for the scanlation before taking it too seriously.