I'm just posting to publicly speculate roles by naming characters, enemies, and bosses in the games (OoT and MM). Really, this is more or less for personal reference, but could help people if they are trying to figure out roles.
Obviously we have Link, Zelda, Ganondorf, and Skull Kid.
Perhaps Majora's Mask itself is a character?
Bosses could play major roles. I'm thinking Dark Link, two people working together as Twinrova, and Phantom Ganon.
Then we have possible enemies.
The only ones that come to me really are Redeads. Could someone possibly continue to play after dying or someone could control people?
I am sure that Zoras and Gorons are possibly out there.
I don't know much about other characters or Majoras mask, so I will speculate items their roles:
Masks
Masks are present in this game, and can be traded. The ones that strike me as possible are Bunny Hood, Goron Mask, Deku Mask, Zora Mask, and Fierce Deity Mask.
Other items
We have Bombs, Arrows, The Master Sword, Sheilds, and so on. There many to list, so I won't list them all. Even the Ocarina of Time can probably be used. In fact, I suspect it will play a major part, especially with it tieing in with the Day/Night Factor of the game. Could someone possibly change the times of the game at will?
Anyways, enough speculation from me for one day, I will post more as the game progresses.