Martin Brisby is the strongest and the 2nd eldest child of Mrs. Brisby and a neutral character in The Secret of NIMH and the (former) main antagonist of its 1998 sequel. He is a tough, pessimistic male mouse. He was voiced by Wil Wheaton in the first film, Phillip Van Dyke as an adolescent in the sequel, Eric Idle as his evil self, again in the sequel, and Phillip Glasser as a reformed adult, again in the sequel.
Martin Brisby is 9-10 years old (by human age) and is the second eldest child of the Brisby family. A running gag is when he has a toxic rivalry with Auntie Shrew because he hates it when she comes over to babysit and argues with her. But he does care for his family, especially when Mrs. Brisby scolds him for being mean to Auntie Shrew, he feels bad for what he did and apologizes. He may somewhat have some similar traits to his father such as personality as well as physical appearance.
Martin is the second child of the Brisby family who dislikes having Auntie Shrew over their house and is not afraid to yell back at her. He claims that he's not afraid of the dark, the farmer, Dragon (the cat), and the Great Owl.
At the beginning of the movie, Martin is very jealous of Timothy Brisby for having been elected by Nicodemus as being the future hero when Martin feels he's much more the hero type than his brother. However, while Timmy is studying with the Rats of NIMH in Thorn Valley, Martin is kidnapped by NIMH researcher Dr. Valentine, who leads further experimentations on him, believing him to be a regular mouse. Unbeknownst to him, however, Martin is already the son of a genetically-engineered mouse, and the treatment he subjects him to makes him turn completely mad and also evil, while also improving his intelligence to almost superhuman level. Having become a threatening mad scientist, Martin betrays Dr. Valentine and takes over the NIMH laboratory and tools, using them to create an army to take over Thorn Valley. His plan is however foiled by Timothy, Martin having turned out to be the threat Nicodemus's prophecy was referring to. Martin is captured and brought back to Thorn Valley, where the Rats manage to heal the damage done by Valentine's experience and turn him back into his normal, non-evil self.
- Martin has gray fur, the same fur color as Jenner.
- He is similar to Skippy in Robin Hood.
- In The Secret of NIHM 2: Timmy to the Rescue, Martin is possibly living proof that the chemicals injected into test rats at NIMH have a small percent chance of driving the subjects insane with increased tendencies of unprovoked aggression and the driven need to dominate others.
- If this theory is correct, then Jenner would have been the first rodent species to suffer from this side-effect.