What anime/manga characters that wear something to conceal, whether partially or fully, their face are the most interesting to you, or do you just like the look of their mask? Did you have any idea who it was? What role did they play? Is there a reason they cover their face?
No Face, from Spirited Away, was an interesting 'masked' character. It served as his face but still resembles a mask and was kept simple yet captivating. He is well behaved, if not provoked, but if provoked, he'll steal and eat anyone and anything in his way in a mad rampage.