Dennis is a lawyer who threatened to take legal action against Jake.
When he had heard his sister Betty was admitted to Sacred Heart Hospital, Dennis went to see her. Just before he arrived, J.D. had convinced Jake to talk to Betty about her decision to be on a breathing machine or not. She declined, and Dennis decided to pursue legal action against Jake. After Betty slipped into a coma it was more difficult to prove their point, so J.D. recommended fleeing. Jake stayed, however, and fought for Betty's wishes, and Dennis realized that Jake was probably in the right and dropped his threat. ("My Faith in Humanity")