Jenner is the very epitome of the worst things suggested by the word "rat."  Ambitious, proud, and greedy, he is willing to take any chance to get what he wants.  He is violently opposed to the rats' plan to move to Thorn Valley, claiming that it will only lead to the rats' demise; that the better option is to continue stealing from the Fitzgibbons family--even if it means declaring open war on they and the rest of the human race.  Whether he actually believes this or is simply trying to oust Nicodemus is hard to say, but he won't let anyone stand in his way and live.  He manages to coerce a fellow NIMH rat, Sullivan, into assisting him with his plan to eliminate Nicodemus when it becomes clear that the Thorn Valley Plan will come to fruition.  But his reluctant and nervous accomplice has a change of heart in the moment of truth, and Jenner makes him pay dearly for it.

Quote:  "The Thorn Valley Plan is the aspiration of idiots and dreamers!  We..." (laughing) "...we were just talking about you."

Origins of the character: In Robert C. O'Brien's book "Mrs. Frisby And The Rats Of NIMH", Jenner is a MUCH different character than he is in the movie.  He is not a scheming, amoral sociopath.  Jenner's simply more of a misguided idealist, who suffers from a touch of greed and a desire for material luxury.  When Nicodemus outlines the divide between he and Jenner over the Thorn Valley Plan, to Mrs. Frisby, he describes it this way: "that was the beginning of an argument that never had a satisfactory ending.  Jenner would not yield to my point of view, nor I to his.  It wasn't that he was lazy and didn't want to work.  He was just more cynical than the rest of us; stealing did not bother him.  And he was a pessimist.  He never believed that we could really make it on our own."

As the divide between Jenner and the advocates of the Plan grows, Jenner decides to leave the rats, with six other companions who are of like mind.  Unlike in the movie, he does not remain until the rats move the Frisby home, and does not manipulate anyone into helping him kill Nicodemus.  He leaves...taking his like-minded companions with him.  In fact, in the book, it's clearly stated that not only is Jenner a close friend of Nicodemus' (since before they were captured and sent to NIMH), but that it was Jenner and Justin who were the first ones responsible for helping figure out how to free the rats from NIMH.  In the book, while Jonathan Frisby is still very important, it's more that he simply released the latch which held the final obstacle in place (along with Mr. Ages' help).  Throughout the NIMH story (all of it), Nicodemus expresses concern about Jenner and his welfare, and what became of him.  All in all, Jenner is not the same character from book to screen.

In the animated sequel: Jenner does not appear in the sequel, as he is killed in the first movie.