It is never shown except for when he is a child in the Fall of Reach graphic novel. But the books DO describe how Master Chief 117 looks. They describe him with dark brown hair and a square jaw with good looking features, but that the most remarkable feature about him is his skins paleness. He is described as having bright white pale skin like a ghost because of all his time spent inside his MJOLNIR armour that he never gets his skin exposed to sunlight often. I picture Chief looking kinda like a pale version of Carter A-259 from Noble Team. If I remember correctly though the chief is also supposed to have a certain look to him, nothing you can quite pinpoint physically but he just has a certain aura to him or look of somebody who is special and mainly lucky or destined for greatness. He had that kinda look to him, however you take that to mean.