Sullivan is the unwilling tertiary antagonist in The Secret of NIMH. He is one of the Rats of Nimh and Jenner's henchman.
He was voiced by the late Aldo Ray.
During the rats' proposal to move the home of a mouse named Mrs. Brisby (as they were friends with her late husband Jonathan Brisby), Jenner plotted to use the transport to kill Nicodemus and make it look like it was an accident, so that Jenner can take over and have the rats remain in the rose bush. Sullivan said it would be risky, but Jenner tells him that it's nothing to worry about.
In the climax, as the rats of NIMH try to move Mrs. Brisby's home as promised, Jenner slashes down the ropes without Sullivan's help, since Sullivan hesitated to do it and tried to stop Jenner from doing it. This causes the gears to fall off, causing the home to land on the mud, killing Nicodemus, and Sullivan feels guilty of letting Jenner do such a horrible thing. When Mrs. Brisby arrives to warn the rats of NIMH that the people working for NIMH are coming to exterminate them if they don't leave the bush, Sullivan believes her, but this causes Jenner to attack Mrs. Brisby, as he still wants the rats to remain in the bush, even if it means to battle the humans to their deaths. Outraged, Sullivan ran to tell Justin about what's happening to Mrs. Brisby and gives him his sword. Enraged, Jenner slashes his former minion in the chest, mortally wounding him.
After Justin defeats Jenner by stabbing him in the chest, he convinces the rats to leave the rose bush, but before he can say anything, Jenner comes back with his sword to make a lethal blow on Justin. Fortunately, Sullivan saves Justin's life by tossing a dagger right into Jenner's back, sending Jenner to fall to his death. Satisfied that he has finally put an end to Jenner's plans for good, the redeemed Sullivan dies in peace.
- Despite being Jenner's minion, Sullivan isn't the secondary antagonist, Dragon is.
- He is referred to by name only in the credits, as Jenner only calls him "my friend."