Joseph Switez is a German doctor and psychiatrist. He is in the midst of a personal research project that involves interviewing citizens of Los Santos on questions of morality.
Notably, at the end of every interview session Dr. Switez hosts, he offers them a mystery meatloaf. Multiple people have commented on its strangeness, even going so far as to bring it to the police for lab testing. The lab results returned showing grey matter in the meatloaf, possibly indicative of some sort of brain matter.
Dr. Switez, though not discovered by any others yet, is a cannibal. He has chopped up a body of a woman named "Isabelle" in his home and made meatloaf with her body parts. It can be assumed that the meatloaf he offers to his interviewees is made of human meat as well.