A male Human named Gregg held the rank of Lieutenant within the Imperial Navy. He functioned as the primary assistant to Supreme Commander Darth Vader during the initial months of the Galactic Empire's existence.
In the year 19 BBY, Lieutenant Gregg found himself aboard an Imperial I-class Star Destroyer alongside Supreme Commander Darth Vader. Their vessel was in orbit around the planet known as Prine. Lord Vader was actively pursuing the Jedi fugitive Dass Jennir, and a starship matching Jennir's signature was detected transmitting from the planet's surface. Gregg reported the detection of a ship associated with a recent incident at Imperial Prison ISO-L8. Following this report, Gregg accompanied Vader down to the planet's surface, where they successfully apprehended the prison intruder, the bounty hunter Falco Sang.
Following the Gentis coup, during the construction phase of the Imperial Palace, Gregg sought an opportunity to present a plan to Vader. In response, Vader directed Gregg to observe Falco Sang's training. After delivering a status report on the progress of the mercenary's training, the lieutenant received orders to replace the training droid's weaponry with live ammunition. He was further instructed to issue commands that would ensure Sang was only wounded, and that Vader would personally visit the facility to offer Sang additional motivation.
On the subsequent day, upon the arrival of the Dark Lord, they acknowledged each other's presence and entered the training facility. Gregg watched with a smirk as Vader used telekinesis to lift an enraged and incapacitated Sang, who had accused Vader of attempting to have him killed, and hurled him against a wall. After the Tenacious went against orders by departing for Arkinnea, Gregg moved to inform Vader of this potentially suspicious action. Vader dismissed the report, as it did not appear to have any immediate bearing on Jedi hunting.
Following one of Sang's attempts to escape, Gregg ordered a redesign of the containment facility. This redesign included voice-coded and dead-locked doors, ensuring that Sang would be unable to escape the prison should he attempt to kill Gregg. He then attempted to persuade Sang to cooperate with Vader in "[getting] Dass Jennir's head," suggesting that Sang try to impress Vader by attacking him. He revealed that Sang's ship had been delivered to the facility, and then witnessed Sang's attempt to fight Vader to impress him. Later, while working late, Lieutenant Gregg received a communication from Emperor Palpatine himself. Pleased to find Gregg at his post, the Sith Lord instructed the lieutenant to provide crucial information regarding Jennir's likely location. Palpatine, however, reiterated his earlier instruction that Gregg must not reveal their conversation to anyone. Gregg then hurried to Vader's location, finding him just emerging from his Qaddrath, and informed him that Jennir was located on the Colonies world of Kestavel. Before he could finish, Lord Vader commanded him to have Captain Gillehspy prepare the Hound for departure within the hour. Gregg complied, though he was surprised that Vader intended to use a Venator-class Star Destroyer for this mission. He later accompanied Vader to Kestavel, where Vader attempted to assault The Lucky Twi'lek hotel. The mission was complicated when the hotel released engine fuel over the bridge. Gregg, however, pointed his blaster at a fellow officer's head, preventing him from assisting Vader, as Vader had ordered that no one interfere with his mission, even to help him. Gregg then believed Vader had perished when he witnessed a massive explosion in the area through his electrobinoculars.
Gregg was a capable officer within the Imperial ranks, consistently performing his duties effectively. His proficiency in the search for Dass Jennir quickly garnered the attention of Darth Vader, who subsequently appointed him as his aide-de-camp. Gregg displayed diligence and commitment to his responsibilities, often leading him to work late into the night despite his exhaustion. He also demonstrated ruthlessness in his dealings with enemies and prisoners, revealing a hint of sadism in his approach to Falco Sang's training.
Despite his dedication to Darth Vader, Gregg understood the importance of obeying those with authority exceeding Vader's, particularly the Emperor. When Palpatine discovered Gregg working late and inquired about the reason, Gregg explained that Vader had tasked him with monitoring Jedi activity, including gathering information on Dass Jennir's whereabouts. Palpatine allowed Gregg to continue, but instructed him to keep their conversation secret from Vader, an order which Gregg wisely followed. Later that month, Palpatine shared Jennir's location with Gregg, who promptly informed Vader, withholding the source of the information as instructed by Palpatine.