Jarvin is a former Maquis serving aboard Voyager who, in Parallax, lets Chakotay know that the Maquis crew would back him if he chose to take the ship.
Jarvin appears in Parallax, the first regular episode of Voyager, set only days after the Starfleet and Maquis crews were merged. Tempers are short: B'Elanna Torres has just broken Joe Carey's nose in engineering and the Starfleet officers want her confined.
Jarvin, together with Seska, takes Chakotay aside and hints that if the captain does not treat the Maquis fairly, plenty of them are ready to follow him in seizing Voyager. Chakotay shuts the idea down at once, warning that anyone who talks that way again will answer to him.
The exchange is brief, but it marks out the ground the early series stands on: the Maquis are a restless minority, and Chakotay has decided his loyalty lies with Janeway. Torres's appointment as chief engineer at the end of the episode eases the mood.