With Gandalf and Saruman being the most involved wizards during The Lord of the Rings, most fans are left wondering what Radagast and the two blue wizards were up to. While much about the blue wizards is left unknown, Radagast has been somewhat mentioned throughout Tolkien's work, including a reveal that Saruman had exploited their friendship to try to gain power over Middle-earth.
Radagast is often described as a gentle soul -- someone who prefers spending time with nature and animals rather than people. While his portrayal in Peter Jackson's The Hobbit trilogy is somewhat over the top, it still shows his love for the forests and his duty to care for them. It's this kindness that makes him so different from Saruman, but also makes him susceptible to lies and deceit.
Radagast Accidently Captured Gandalf
When Gandalf arrives in Rivendell for the Council of Elrond, he explains why he's been missing for so long. It's revealed that Saruman has sided with the Dark Lord and that Gandalf had been held prisoner at Isengard for a long while. But Gandalf also mentions how it was Radagast who sent him there, not knowing the evil plans his friend, Saruman, had been preparing.
Just months before Frodo departed from the Shire, Saruman found Radagast and asked him for his aid. Using Radagast's birds, he had the wizard search the northern parts of Middle-earth for Gandalf, claiming that he simply needed his help to fight Sauron. Radagast thought nothing of it and searched for months until finally he and Gandalf crossed paths near Bree. After sharing warnings of the Nazgûl, he sent the gray wizard on his way to Isengard, along with several birds to report news about the war. The two departed, with Radagast unknowingly sending Gandalf to be captured.
Radagast Was Gone During The Lord of the Rings
Radagast just so happened to send a giant eagle over to Isengard, where Gandalf was lucky enough to be rescued and returned to Rivendell. It's likely that after this the eagle returned to Radagast and reported on Saruman's betrayal, causing great guilt in the friendly wizard. But after this, Radagast is never seen or mentioned throughout the rest of The Lord of the Rings, and his actions after the war are completely unknown.
It's entirely possible that by accidentally aiding Saruman, Radagast thought it best to keep out of everything. It's even mentioned that scouts were sent to Radagast's home in Rhosgobel, only to find him missing. And whether he helped fight the war or not, he would eventually have to leave Middle-earth with the elves. The wizards were sent to Middle-earth to defeat Sauron, and so with that completed, his time was at an end. He could opt to stay with his forests and animals if he wanted, but his powers would dwindle over time, and he'd succumb to the same fate as all men.
While little is known about Radagast compared to most The Lord of the Rings characters, the small details all paint a clear picture of who he was. Much like the Hobbits, Radagast found comfort in nature and living a simple life, which makes it all the more depressing that Saruman took advantage of his kindness. And it may very well explain his absence from the story.