In 2151, he was the captain of a group of Ferengi pirates that included Krem, Muk, and Grish. The pirates used a "Trojan horse" device, disguised as an ancient artifact, to knock out the entire crew of the Earth Starfleet ship Enterprise.
Once the device was brought aboard Enterprise and its crew were rendered unconscious, Ulis and his crew boarded the ship and began looting it. He awakened a restrained Captain Jonathan Archer and questioned him as to the whereabouts of the captain's vault, unaware that the Earth ship did not contain one. Seeing that Ulis and his men relied on profit, Archer began to play along, all the while building on the Ferengi's greed and mistrust for each other.
Unbeknownst to the Ferengi, however, the ship's chief engineer, Commander Tucker, was not affected by their device, and was able to revive T'Pol before being captured. Archer and Tucker continued to play on the Ferengi's greed, and Tucker promised to take them to the ship's "vault". Ulis ordered Krem to guard Archer while he and the others followed Tucker. The engineer led the greedy aliens through a maze of corridors before finally arriving at the "vault" - which turned out to be a trap in which T'Pol ambushed the Ferengi with a phase pistol. With their plan foiled, Captain Archer put Ulis, Muk, and Grish in restraints, placing Ulis' ship under the control of Krem. (ENT: "Acquisition")