Tessic was a member of an alien species and the leader of a colony that mined deuterium in the 22nd century.
Enterprise NX-01 visited the colony in 2152, seeking to buy or trade for deuterium. Tessic at first wanted nothing to do with the visitors and asked them to leave. After negotiations with Captain Jonathan Archer, Tessic agreed to give him the deuterium in exchange for medical supplies, power cells and assistance in repairing the colony's extraction pumps, but told Archer that they had to leave in two days. Tessic was trying to hide the fact that Klingon marauders were preying on the colony, taking the deuterium, leaving just enough for the colonists to survive.
When the Klingons arrived, Archer and his crew witnessed the Klingon marauders abusing Tessic. He told Archer that when he and others had tried to rebel, it had resulted in the deaths of eight colonists. The crew helped Tessic and his people to fight off the Klingons, whom they trapped in a flaming deuterium field. The Klingons left the colony in peace. (ENT: "Marauders")
Tessic was played by actor Larry Cedar.
In the final draft script of "Marauders", Tessic was initially described thus; "He's had to bear the responsibility of protecting his people for a long time, and it's made him a bit mistrustful... skittish about outsiders." More indication of how long Tessic was familiar with Klingon leader Korok was established in the script than there is in the final edit of the installment, the teleplay commenting, "He's spent years fearing this man."