Marty Fisher is the brother of Veronica and the son of Carol Fisher. He makes an appearance during the episode Three Boys. He is seen in the closed bathroom stall next to Lip and Karen having sex and makes a sarcastic comment about it. He lights a match at the end of the episode which would indicate that he tries to burn down the Alibi. It is unknown what happens to him as he has not been mentioned again as of the end of Season 4.
Marty has a severe case of Tourette syndrome, and a violent temper, causing him to yell out curse words without warning. Veronica also claims he has bipolar disorder. His keen liking of fire would also suggest he is a pyromaniac.