Nicodemus is probably the only refugee from NIMH older than Mr. Ages.  And while Mr. Ages developed a talent for medicine and chemistry, it would seem that Nicodemus managed to tap some unforeseen latent magic in the world, and has become a wizard.  He regards both the late Jonathan Brisby and (surprisingly for a rat) the Great Owl as dear friends and, to honor his friendship with Jonathan, he would gladly move Heaven and Earth to help Mrs. Brisby.  He is the leader of the rats of NIMH, though in the movie we often see him sequestering himself within his own chambers, keeping a log of his experiences, and pondering a great deal upon his friend Jonathan (and the rest of the Brisby family) and upon the Thorn Valley Plan.  However, not all of the rats are satisfied with his rule, and Jenner in particular might be looking to usurp Nicodemus' authority and undermine the Thorn Valley Plan.  At any expense.

Quote: "We can no longer live as rats--we know too much."

Origins of the character: In Robert C. O'Brien's book "Mrs. Frisby And The Rats Of NIMH", there is no character which is so very different from how it appears in the first movie than Nicodemus.  Unlike the wise old wizard seen in the movie, in the book Nicodemus is a younger, more virile leader of the rats, and definitely not a wizard (nor is there any magic in the book whatsoever).  He is described as having a scarred face, but that he "spoke graciously, with an air of quiet dignity, and Mrs. Frisby noticed two more things about him.  First, the scar on his face ran across his left eye, and over this eye he wore a black patch, fastened by a cord around his head.  Second, he carried a satchel--rather like a handbag--by a strap over his shoulders."  Nicodemus is still a very wise and noble character, and also shows great cleverness and intelligence, and a high sense of morality.  Though, at times (especially in the sequel book "Racso And The Rats of NIMH", written by O'Brien's daughter, Jane Leslie Conly), he also could be stern and very discerning.  He is the prime proponent of the Thorn Valley Plan, freely expressing his distaste at the rats having to live by stealing from the Fitzgibbons or any other humans, but noting that the rats of NIMH cannot live either like ordinary rats, or in the manner of humans.  That they must set their own destiny and disdain the luxuries they set up for themselves in the colony under the rose bush. 

He also, in the books, has a great fondness and concern for his good friend, Jenner.  Unlike in the movie, Nicodemus and Jenner were always friends...even before their capture by NIMH.  And after Jenner and his six friends depart the rose bush colony, after the decision to move to Thorn Valley is certain, Nicodemus assumes a constant concern for his old comrade...always eager for news of his fate, and hoping for the best.  Additionally, while he and the other rats (and the mice) who were experimented on at NIMH didn't totally enjoy the experience, Nicodemus is much kinder and more forgiving of the scientists than he is in the movie.  He even notes that the assistants of Dr. Schultz, the lead scientist and researcher at NIMH, treated the rats with a fair amount of gentleness and even sympathy (particularly a young woman named Julie), and that the rats, overall, were treated "well enough".  For fans of the movie, of course, these facts will seem quite out of sorts.

In the animated sequel: Because of the events at the climax of the first movie--where Nicodemus is killed when the Brisby home falls on and crushes him (a disaster set in motion by Jenner)--Nicodemus does not appear in the sequel, except in a sequence of flashbacks during the prologue (where the narrator not only misquotes him, but refers to him as a "prophet", which a couple of other characters do during the movie.  And which he is not.)