Jenner is an indirect character in the novel Mrs. Frisby and the Rats of NIMH, since he only appears in scenes with a flashback format. Although he doesn't have a physical presence, his influence is still strongly felt by the rats, and this sense is conveyed in many ways other than just the memories of him. Since Jenner was opposed to "the Plan," the manner in which he left may seem ironic, as, ultimately, it helped the rest of the rats accomplish their mission to leave for Thorn Valley.
Jenner had always relied on and promoted the old ways of rat culture, which were basically to hide and steal from humans. He was a practical type of character who felt that change was detrimental to their way of life. His stubbornness proved to be his greatest fault, and he paid for it with his life. He left the safety of the group to steal electricity from humans and was killed during the process, along with his followers. The ironic part is that his death alerted the rest of the rats in the colony to human threats such as extermination, and they left for the Valley and survived.