Who is Eli Mayhew from While I Was Gone and what is their importance?

Eli appears in two different versions in the novel. First he is a young man living in the group house in Cambridge. In the house he feels like something of an outsider because of differences in personality and career focus as compared to the other residents. Unlike the other residents, he abstains from drugs and alcohol. The second version of Eli is a middle-aged man who meets Jo again when Eli needs the services of a veterinarian.