DeMarco was born and raised on the colony of Algolis. He joined the UNSC Marine Corps at some point and eventually became an ODST, fighting numerous campaigns and battles across UNSC-controlled space. Despite his brash attitude and often arrogant personality, ONI analysts found him to be a brilliant tactician and quick-thinking strategist, and he was nominated for induction into the SPARTAN-IV program.
DeMarco served as fireteam leader of Majestic during several deployments in the wake of the Human-Covenant war. At some point before 2558 DeMarco and the majority of Majestic were stationed in Rio de Janeiro on Earth. They were then transferred to UNSC Infinity for deployment to the recently discovered Shield World Requiem. Upon disembarking, they are greeted by Roland, the ship's AI, who ordered them to line up and await an escort. As Majestic complied, DeMarco spotted an imposing brunette marching up to them. Though Madsen wanted to make a pass at her, DeMarco stated he saw her first. But then he discovered more than he bargained for, as the brunette was Commander Sarah Palmer, leader of Infinity's Spartans. DeMarco quickly fell back in line and ordered his team to salute the superior officer. Palmer then ordered them at ease and to follow her. Hoya chuckled at DeMarco's humiliation, whereupon DeMarco told him to be silent.
Second Battle of Requiem
- Main article: Second Battle of Requiem
On February 7, 2558, DeMarco led Majestic and a contingent of Marines to secure a foothold for UNSC forces on Requiem. In the days that followed, DeMarco and the rest of Majestic were deployed once again to Requiem alongside Fireteam Crimson to clear a pair of Forerunner facilities of Prometheans. Commander Palmer challenged the two fireteams to see who could complete their objective first, a challenge which Majestic swiftly accepted and then lost.
Majestic would later be sent to engage Prometheans in one of Requiem's desert areas. When the coast seemed clear and Madsen commented that the Prometheans appear at their leisure, Knights materialized out of nowhere, and shot Madsen in the shoulder. This prompted Hoya to break cover and go berserk. Majestic managed to return to Infinity intact, albeit with Hoya on a stretcher. They are greeted by Palmer, who asked what happened to Hoya. DeMarco simply responded, "Hoya happened to Hoya." Palmer then dismissed him.
After the disappearance of Dr. Glassman into a Forerunner artifact, DeMarco, Madsen and Palmer escorted Dr. Halsey aboard Infinity so that she could examine a captured piece of Forerunner technology. As they take a vertical train to the engine room, DeMarco asks Madsen why they are guarding an old lady. Madsen explains that Halsey used children in the SPARTAN-II Program, the legendary Master Chief among them. As DeMarco asked out loud why she would experiment on children, Halsey interrupted them, stating that she taught them how to be silent, and then how to be Spartans.
Upon arriving in engineering, Halsey began diagnostics, shortly before a ripple of orange energy washed over them and the Spartans raised their weapons on her in alarm. After Palmer motioned them to stand down, Halsey explained that the artifact is communing with the ship's engines, the only Forerunner technology installed, as well as the other systems and transmitting to somewhere on Requiem.
After that Majestic was once again deployed to Requiem alongside Crimson, this time in search of Jul 'Mdama's Covenant leader, Jul 'Mdama. Although Majestic didn't find him, they were called to Galileo Base to assist Crimson in the defense of the lab against a Covenant assault. DeMarco appeared to be a fairly adept marksman, being able to take out multiple moving Promethean Crawlers in one attempt.
As Crimson provided cover fire, Majestic was to retrieve a package called "the Didact's gift" and bring it back to Infinity. However, they became pinned down by a Phantom and a couple of Banshees. DeMarco ordered his team to make for the Pelican but Madsen stopped him, warning that they'd be shot down if they tried. Gabriel Thorne then passed the package to Hoya and ran off. DeMarco ordered him to get back in formation as Thorne jumped off the precipice and commandeering a Banshee. Tedra Grant and Madsen went to back him up, assuring the others that Thorne's plan will work. They then destroyed the Phantom and returned to Infinity.
Sometime afterwards, Thorne was transported to Requiem after carelessly touching the artifact, Majestic was sent in search of him. They found him and Glassman, about to be killed by Gek 'Lhar, Jul's second-in command and instead killed him. They then looted the Sangheili's body. DeMarco was disgusted to find dogtags, some of which belonged to Spartans. Thorne then took Gek's active camouflage unit, DeMarco his energy sword and Madsen a holographic map.
As Majestic escorted Glassman to a Pelican to return him to Infinity, they receive a communication from Captain Thomas Lasky, who orders them to rescue Halsey, who was kidnapped by Jul and had a kill order placed on her by Admiral Serin Osman. Majestic made their way to The Librarian's shrine, where Thorne gets the Covenant's attention before Grant, Hoya and Madsen fired from the high ground. DeMarco aided the fight with his energy sword and ordered Thorne to proceed inside and rescue Halsey. As soon as the guards were all dead, Majestic went inside as well. They found Palmer, who was there to assassinate Halsey, and a host of Promethean Knights summoned by Jul and engaged. Unfortunately, as they cleared out the Knights, one lone Knight teleported Jul away with Halsey as his hostage. After briefly screaming, Palmer then demanded to know as to Majestic's presence. DeMarco explained that Lasky sent them on a rescue mission. Palmer retorted that Halsey's a traitor. However, Thorne didn't believe that, as he showed everyone the orange half of the Janus Key Halsey passed him.
Battle of Ealen IV
- Main article: Battle of Ealen IV
- “Actually... I'm good where I am.”— Paul DeMarco's last words
In March 5, 2558, DeMarco, Fireteam Bailey, and Fireteam Jackknife were selected to be the security detail for Lord Hood and Thel 'Vadam as they entered a diplomatic peace talk with the Jiralhanae Chieftain Lydus on Ealen IV. As the Ambassadors were having their discussion, the meeting was ambushed by Jul 'Mdama's Covenant. DeMarco and the other Fireteams were able to clear a patch for the parties to successfully escape the heavy Jul 'Mdama's Covenant Assault and managed to safely secure everyone into an old parts depot. From there, they traveled to a redoubt, destroying an assailing Covenant Lich along the way.
During a sweep of the redoubt, DeMarco and Fireteam Bailey discovered an old Covenant tug that they could use for their escape. However, the tug was too small to carry everyone and ten Heavy Jul 'Mdama's Covenant Cruisers were bearing down on the redoubt. DeMarco stayed behind and used an old AA Cannon with no targeting system to defend the tug as it traveled back to the Infinity, successfully stopping Jul 'Mdama's Covenant from destroying the tug but sacrificing his life.
- A Spartan with the same face model as DeMarco but in standard black and gold armor appears briefly behind Sarah Palmer in the post-credits cutscene of Halo 4.
- In addition to a BR85 Heavy Barrel Service Rifle, DeMarco also uses an Energy Sword, looted from Gek 'Lhar's body.
- DeMarco is the second known Spartan-IV to die during a final stand, the first being Edward Davis.