Gandalf (also known as Olorin, Mithrandir, the Grey Pilgrim, Lathspell, Stormcrow, and several other names) is one of the Istari (wizards) in J.R.R. Tolkien's stories about Middle-earth. Before his climatic fight with a Balrog in the Mines of Moria, he is known as Gandalf the Grey; when he returns (presumably from death), he is known as Gandalf the White.
Gandalf is smart, experienced, and politically savvy and it is his planning, knowledge, and persuasion that spur people to do the things that need to be done so that evil doesn't take over the world. He's also, at the same time, a humble person who is content to play a supporting role as an advisor, rejecting worldly power as contrary to his duties. This caused him to reject the leadership of the White Council against Galadriel's wishes. He is the keeper of Narya, one of the three Elvish Rings of Power, which has a red stone and is associated with fiery things such as hope and courage. At the end of The Return of the King, Gandalf ships out to the Undying Lands with fellow ring-bearers Galadriel, Elrond, Frodo, and Bilbo, and is never seen again in Middle-earth.
In Suefic, he has two main roles: either Designated Misogynistic Bastard or discoverer of the Sue's talents and the person who recommends her for a place in the Fellowship or Thorin's Company. Probably fortunately for us all, he doesn't feature too often in bad slash, although he does make a fair number of appearances in crackfic.