Joe is a member of the Civil Border Patrol, who alongside his best friend Josef, a Russian immigrant obsessed with America, they hunt "illegal immigrants" in order to deport them back to their home countries. However, most of the time the "illegals" they pursue are naturalized US citizens of mainly Hispanic origin.
Events of GTA V
During their missions, Joe and Josef employ Trevor Philips twice to help them hunt down supposed "illegal aliens". Trevor, a Canadian immigrant himself, dislikes the duo, but assists them because of the monetary reward. After his second outing with the duo, Trevor re-encounters Manuel, one of the men arrested during his second tour with the Patrol, who is a legal United States citizen as his ancestors have lived in San Andreas for over 200 years, well before it was incorporated into the United States. Manuel reprimands Trevor for his actions and, in a regretful fit of rage, Trevor tracks down Joe and Josef to a nearby farm and attacks them. After Josef is killed, Joe tries to escape in a Fieldmaster, but Trevor stops him and kills him as well.
Besides English, Joe apparently knows how to speak Russian, since he is capable of understanding Josef.
He reveals in the mission An American Welcome that he actually works as a Park Ranger, and that he only pursues "illegal immigrants" in his spare time.
In Minute Man Blues, It is likely that the SAPR fired Joe, due to his illegal activities, and as a result Joe and Josef had their Park Ranger and tasers, confiscated.
This is hinted by the fact that Manuel stated that he was going to report them, and in their confrontation with Trevor, their Park Ranger is nowhere to be seen and they're using lethal weapons, such as the Sawn-off Shotgun.
He also reveals that he has a brother who has three children and used to be gay until their parents "set him straight".