If ever a mouse demonstrated courage and strength of character, it was Jonathan Brisby. One of the escapees from NIMH, he is regarded as a hero by the rats, not only for the part he played in helping them escape, but also many untold exploits thereafter...until the day when his luck ran out, and he was killed by the Fitzgibbons' cat, Dragon. Given the changes he, Mr. Ages and the rats went through during the experiments at NIMH, he chose not to tell his wife or children about this, for fear that she would not understand how he (as well, most likely, as their children, Martin, Teresa, Timothy, and Cynthia) remained young while she grew old.
Jonathan Brisby is only seen in the film during a magical sequence and a tale, provided by Nicodemus to Mrs. Brisby. Though many of the characters talk about him a great deal, usually with reverence and love.
Origins of the character: In Robert C. O'Brien's book "Mrs. Frisby And The Rats Of NIMH", Jonathan Frisby is essentially the same character. There are almost no changes...though we learn more about him in a longer series of tales which the character Nicodemus tells to Mrs. Frisby, including that Jonathan is essentially the leader of eight mice (which is three less than the number given in the movie). Jonathan attracts the attention of the rats as they are working out their escape from NIMH, and offers to help if the rats will allow the mice to accompany them. When one of the rats protests about freeing the mice, claiming that they'll only slow the rats down, Jonathan replies "no, we will not. Only open our cages when you go, and we will make our own way. We won't even stay with you, if you prefer."
In the animated sequel: Jonathan Brisby does not appear in the animated sequel, but for a replay of the magical sequence seen in the first movie, and that he is a constant weight hanging over his son Timothy's head (and even his son Martin's) as they are growing up, and Timothy is trained at Thorn Valley and groomed to eventually live up to his father's legacy and be a hero. Of course, there is a big impressive statue of Jonathan in the center square of the Thorn Valley colony, which only rams that point home that much more for poor Timothy every day he spends in Thorn Valley.