In my Fanfiction "The Legend of Zelda : Winged Ones" the Fierce Deity AKA the Oni Mask reflects the form of a lost race of people (makes sense, all the other masks that change your form are a race in the game) and Majora had removed the angel half in hopes that the remaining pure demon half would consume Link's soul. (it is suspicious that he gave you this mask, there is also mention of the mask perhaps being as evil as Majora's) In my story the angel is returned to the mask and Link takes on the form of a Tenshioni with wings and blue eyes.

To learn more about/to read this fanfic go here http://wingedones-zeldaff.deviantart.com/