Thomas Kelly

"Took your sweet time, Marine?"

- Sergeant Kelly's first greetings on the player

Master Sergeant Thomas Kelly is a major character in Doom 3. A strict military sergeant who expects a lot from his soldiers, he provides the player his objectives before later serving as a secondary antagonist.

Background
A decorated veteran of the UAC Marine Corps, Sergeant Kelly — or Sarge as he is known to his troops— is the ranking commanding officer on Mars Base, at the top of the enlisted chain of command on the Mars Base (Marine HQ).

Sergeant Kelly is known to be a no-nonsense commander, as he has little patience for those who are unable to carry out their orders, especially at their fullest potential. As a result, he has no qualms to assign such soldiers to the worst duties as punishment (i.e. in the solid waste facility). Despite being cold, he is very well respected for his duties, leadership, and discipline, as he only wants his soldiers to get better. Thus, he maintains a very fair treatment towards his men, giving good word to those who fulfill their duties and even motivating those who have the potential to get better.

Sergeant Kelly is known to keep a stoic mindset when things are at their worst. His sharp tactical mind allows him to flawlessly direct his squads to a brilliantly conceived battle tactic. There are those who would call Sarge a machine and those who might think otherwise won't be able to deny.

Outside the military, Kelly spends his personal time doing wildlife hunting. In one instance, he managed to hunt a grizzly bear with his double-barreled shotgun. Kelly appears to be very proud from this hunt as he has a head trophy of the same bear mounted on the wall in his office, as well as having the shotgun he used and a photo to commemorate the occasion.

Doom 3
Once the transferred Marine arrives to Mars, Kelly urgently orders him to report himself to Marine HQ where he was waiting. Once the Marine arrives to HQ, Kelly expresses brief annoyance for taking so long. He eventually shrugs it off and provides the Marine his first assignment, to locate a missing scientist who was last seen at the Old Communications Facility and bring him safely back to HQ for interrogation. Sergeant Kelly then sends a Sentry Bot to guide the Marine.

Not long after the Marine locates the missing scientist, the demonic outbreak begins, killing (and possessing) Dr. Ishii and along with almost everyone on the entire base. Sergeant Kelly, now in the Delta Labs, orders all remaining Marines to head back to HQ for further orders.

Once the Marine makes it back to HQ, he is the only one alive. He then approaches the communications monitor, establishing a connection with Sergeant Kelly. Surprised and relieved to see the Marine alive, Kelly gives him his next mission: to link up with Bravo Team. Kelly then marks the Marine's path to the Alpha Labs as the quickest way to get to them.

Kelly spends the next several levels monitoring the Marine's progress, warning him of some potential dangers he will face and reminding him how close he is to Bravo Team. Eventually, once the Marine makes it to the Enpro Facility, Bravo Team gets wiped out, leaving their operational status unknown. Kelly gives the Marine a new mission: locate the fallen Bravo Team and obtain the transmission card. Once the Marine arrives where Bravo Team has fallen, Kelly then asks the Marine to take the card to the Communications Tower to have the distress signal sent to Earth for more reinforcements. Kelly also asks the Marine to be wary of Elliot Swann and Jack Campbell, as they are attempting to shut off all communications from Earth, potentially hindering their progress of getting the transmission sent.

By the time the Marine makes to the Satellite Control room, he prepares to have the transmission sent. Swann, however, suddenly establishes connection with the Marine, urges him not to, as Swann thinks it's better to keep the situation quarantined on Mars until it can be contained.

If the Marine sends the transmission, Kelly commends him for his efforts and asks him to rendezvous at the Delta Labs, as Kelly also discovers the main portal in the Delta Labs was how the invasion started.

If the Marine cancels the transmission, Kelly will angrily blast him for insubordinate conduct, and firmly asks him to return back to the Satellite Control Room to send the transmission before being inexplicably cut off.

At this point, the Marine is not been contacted by Kelly. By the time the Marine arrives the Delta Labs, Kelly is nowhere to be found, but a video-link up request is pending at a communication terminal, forcing the Marine to set up connection. Kelly appears, only in a recorded message, stating their original rendezvous point has been overtaken by demons, and thus has been compromised. He instead sets up an alternate route nearby in the Delta Labs, asking the Marine to rendezvous there.

It is eventually revealed that Kelly has converted over to the demons' side. He ambushes Campbell and Swann in Delta Labs, gravely injuring Swann in the process and forcing Campbell to leave him behind while he goes after the Sarge. Campbell, unfortunately, succumbs as well, and dies before having his weapon acquired by Kelly. Once the Marine discovers the fates of both Swann and Campbell, Kelly then taunts the Marine that he will come after him next.

The Marine finally meets up with Kelly in the CPU Complex. Seeing that his former commanding officer is now a large tank-like cybernetic monster, armed with Campbell's BFG9000, the Marine has no choice but to fight and kill him. The Marine slays Kelly, finally putting the once proud military veteran out of his misery.

Good or Evil?
The exact circumstances of Kelly's transformation are uncertain. Some players affirm that Sergeant Kelly was working with Betruger and the demons since the beginning. This is substantiated by the fact that Kelly insisted in contacting Earth to bring the armada to Mars, maybe knowing that the fleet would be taken over by the demons.

However, a careful review of Master Sergeant Kelly's radio dialogue from the start of the invasion until the player reaches the comm tower strongly suggests he was never initially in league with Betruger, nor did he had any prior knowledge on Betruger's plans before the invasion began.

Kelly's statements and orders were concurrent with his leadership experience. Despite having very limited intel on the opposing enemy forces, Kelly acknowledged he was dealing with a full-scale attack by an unknown enemy force; nothing was known about their numbers and capabilities. All that was known to him amid the chaos was that his troops were being overwhelmed, the units were cut off from one another and unable to regroup or fortify their positions, and most of the base was already in enemy hands.

Both Kelly and Swann had reasonable justifications in their dispute; Kelly urgently needed the reinforcements from Earth in order to prioritize the base's security in Mars and ensure a tactical counteroffensive to engage their enemy, while Swann disagreed on the basis of not knowing what they were up against and keeping the threat quarantined to Mars to ensure safety for Earth.

Had Kelly and Betruger been allies, he or Betruger would have sent the signal themselves the instant the invasion began, instead of going through the trouble of asking the Marine to have the distress call sent. Furthermore, Kelly never personally been to Hell, and only discovered later that the source of the demonic invasion was from the main portal in the Delta Labs. Therefore, he would have no reason for being in league with Betruger before the invasion.

Trivia

 * In the BFG Edition, Kelly has brown hair.
 * Going against Kelly's orders by cancelling the transmission, as well as fighting him in his Sabaoth form may be a reference from the original Doom's backstory, where the protagonist revolted against his superior for giving him orders to shoot civilians.
 * In the French version of Doom 3, Sarge's "final warning" is in English.