Thomas Kelly

"Took your sweet time, Marine?"

- Sergeant Kelly's greet 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 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 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 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 can no longer 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 at him 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 reports to all Marine units on Mars that the base is going through full-scale attack by an unknown enemy force. Now in the Delta Labs, he orders them to head back to HQ for further orders.

During this process, Kelly tries to obtain as much intel as he can. His troops are being overwhelmed and cut off from one another, preventing them to regroup or fortify their positions. Now in desperate need for reinforcements, Kelly eventually discovers only one Marine team left operational: Bravo Team. He orders them to have a distress call sent to Earth and continue to engage against the hostile forces.

Meanwhile, the Marine successfully makes it back to HQ, being the only soldier present. 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 encounter 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 contacts the Marine, and urges him not to, as Swann thinks it's better to keep the situation quarantined on Mars until it can be contained.

The Marine then decides if he should either send or cancel the transmission:

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. (This decision is eventually negated by Dr. Malcolm Betruger, who will fulfill Kelly's request himself.)

At this point, Kelly no longer contacts the Marine. 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 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 fights and then kills him, finally putting the once proud military veteran out of his misery.

Trivia

 * In the BFG Edition, Kelly has brown hair.
 * Kelly's affiliation throughout the game and the circumstances surrounding his transformation are left ambiguous.
 * Some players affirm that Kelly was working with Betruger since the beginning, substantiated that he insisted in contacting Earth's Fleet to fulfill Betruger's goal of spreading the invasion to Earth.
 * Other players believe that Kelly was eventually mind-enslaved by Betruger, and his desperate attempts to have a distress call sent to Earth was based on his own military and leadership experience, as he knew he was fighting an otherwise hopeless battle.
 * 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.