Jenner only appears in Nicodemus's flashback, but he looms large in the story nonetheless. That's because he flat out rejects the Plan, which is a pretty big deal for the rats of NIMH. In that sense, he represents an alternative view to that of Nicodemus. While ol' Nicky believes that in order to survive, the rats have to become very un-rat-like and go live on their own, Jenner believes they should stay true to their rat nature and continue to live the life they always have—before NIMH, that is.
At first, Jenner and Nicodemus feel the same about everything: they want freedom, they hate the uncertainty of NIMH, they excel at reading and debating. However, as the months pass after the escape from NIMH, Jenner starts to have second thoughts about the Plan. When the other rats converse about the need to start a new, groundbreaking rat society, Jenner starts getting gloomy and sulky. His deal is simple: he believes that it doesn't make sense for the rats to start over again in Thorn Valley, when they already have everything they need:
"I say, why start from nothing if you can start with everything? We've already got a civilization." (22.56)
When he puts it this way, it seems like he kind of has a point, huh? While they are scrounging for survival, that's actually nothing new for rats. In fact, that's pretty much par for the course. The rats have come so far and worked so hard to get to where they are, and that would be good enough for most rats. It's certainly good enough for Jenner. Will freedom really be worth losing all of this? Jenner definitely doesn't think so.
In these two rats, we have two very different character types: if Nicodemus is a romantic, with big ideas about the future and independence, Jenner is a realist, practical to the core. And if you need anymore proof that Jenner is brutally practical, think about when he says, "People have been trying to exterminate rats for centuries, but they haven't succeeded… Let them try. We'll figure out where they keep the dynamite is and use it on them" (22.54). Jeez, buddy. You're starting to sound a little… dangerous.
In the end, Jenner's reliance on stealing and humans gets him killed. He and his friends are electrocuted in the hardware store, where it appears that they were trying to route electricity through a motor for their own use. (Remember the headline about "MECHANIZED RATS?") Even though Jenner doesn't make it, the rats learn about his death from this newspaper story and get a heads-up on the exterminators. In a way, Jenner's death (and the fact that Mrs. F overhears the humans talking about it) enables the other rats to escape and make it to the Valley, where we're hoping they'll find a happier end than he did.