One of the less noted characters in the film, Sullivan comes across as something of a temperamental pushover.  On one hand, in the council chamber of the rats of NIMH, he's passionate and fiery in his opinions, and is not afraid to voice them vigorously.  However, he also ends up as a somewhat reluctant and nervous accomplice of Jenner, in that rat's plan to eliminate Nicodemus.  Jenner fully manipulates Sullivan's weaknesses until, at the last moment, Sullivan has a change of heart, and does the right thing.  And Jenner makes him pay dearly for it.

Quote: "We have urgent problems of our own!  Let the lower creatures fend for themselves!"

Origins of the character: In Robert C. O'Brien's book "Mrs. Frisby And The Rats Of NIMH", Sullivan is still very much a character of minor note.  He is only mentioned once, though it is in a much different situation than in the movie.  Sullivan is a "great friend" of Arthur, the engineer of the rats' community (a character who doesn't appear in the movie, and isn't mentioned in it) and, "like him, had a particular interest in engines and electricity."  However, no mention is ever made of any cooperation with Jenner, nor of being manipulated by Jenner.

In the animated sequel: As is seen in the first movie, Sullivan is killed by Jenner when he refuses to help him kill Nicodemus.  Therefore, he does not appear in the sequel.