Aiwendil bird-lover, best known as Radagast,
adûnaic for 'tender of beasts', the Brown, one of the five Istari
sent to Middle-earth to contest the power of Sauron. He was a Maia of
Valinor, and before the time of the mission, nothing is said of him. In
the council of the Powers that was to decide which Ainu were going to
be sent in aid of the free peoples, three were chosen: Curomo, Alatar
and Olorin (Gandalf). But two were later added; Aiwendil was one of
these, taken by Curumo at the request of Yavanna.
After his Arrival to Endor in 1000 TA, Radagast was wandering Middle Earth, developing relationships with its lower creatures, the animals. He gained wisdom in his own way during this period, through reading the growth and memories of the plants and beasts. He settled in Rhosgobel, said to be by Greenwood the Great, later Mirkwood, on the east shore of the Anduin and between the Carrock and the Old Forest Road.
As an Istar he bore the image of an old man, aging very slowly, and always wore a brown cloth and tunic.
Mostly interested in the creations of Yavanna, he did not succeed in his mission. Radagast failed in the Quest the Valar assigned the Wise, but not like Saruman, since it would be more correct to think that he didn't care for the events of the free peoples. He had little part in the War of the Ring, and unwittingly helped Saruman capture Gandalf at Orthanc, but later provided support by sending the eagles to the aid of Gandalf, and also, some suspect, in the last Battle of the Morannon.
In the Council of Elrond, Gandalf describes him as "...a worthy wizard, a master of shapes and changes of hue; and he has much lore of herbs and beasts, and birds especially are his friends." He then affirms the wizard's loyalty with another statement:
" 'At first I feared, as Saruman no doubt intended, that Radagast had also fallen. Yet I caught no hint of anything wrong in his voice or his eye at our meeting. If I had, I should never have gone to Isengard, or I should have gone more warily. So Saruman guessed, and he had concealed his mind and deceived his messenger. It would have been useless to try and win over the honest Radagast to treachery. He sought me in good faith and so persuaded me.' "
Many conjectures state that he could have been allowed back to Valinor with time, or that he simply fell in love with Middle-earth, becoming a mysterious character, as Tom Bombadil.
References: Silmarillion , Unfinished Tales