In the book, Mrs. Frisby and the Rats of NIMH, Nicodemus is the leader of the Rats of NIMH. He is an older rat that is blind in one eye, and is best described as an idealist. He is the primary architect of the Plan of the Rats of NIMH.
In the movie The Secret of NIMH, he has mysterious abilities in the use of magic. He was also assasinated by Jenner in a plot to overthrow the Plan.
Oh, yeah, I suppose I should also mention that Nicodemus is also, in the Bible, a member of the Sanhedrin that defended Jesus to the Pharisees. See John 3, 7:50, and 19:39.