Matt Miller, a 16-year old hacker and expert cyber criminal, is the leader of the Deckers, a Steelport gang that controls the upper west side of the city. The Deckers were let into the Syndicate by it's leader Phillipe Loren because he couldn't figure out how to work his cell phone. Miller is one of the main villains in Saints Row: The Third yet is one of the main protagonists in Saints Row IV.
At the beginning of Saints Row: The Third, Matt is responsible for stealing the Saints assets, leaving them broke and stranded in Steelport. As The Boss starts to encounter Miller's Deckers, Miller will call and antagonize the boss (without revealing his identity). He further meddles with the Saints by using his hacking skills to remotely take control of a helicopter, making Shaundi miss a sniper shot aimed at Killbane.
After Loren's death, Miller accepts Killbane's hostile takeover of the Syndicate and sets to work eliminating their new threat from the Saints. Miller sends his Deckers to capture and kill former FBI agent Kinzie Kensington, whom he had formerly gotten thrown out of the service by hacking the FBI database and setting her up as selling classified information to the Israeli Mossad, and working as a dominatrix on the side. She is recused by The Boss and joins the Saints to work against Miller. Miller amps up his attacks against Kinzie and the Saints, going as far to hacking into Kinzie's personal usenet and discovering her plan to do the same thing to him.
After a campaign of gathering resources for Kinzie, the boss invades the Decker's usenet in the mission "http://deckers.die" by importing his consciousness directly into the system. Miller immediately tries to shut The Boss down by running various anti-virus programs which sends virtual enemies to murder the boss inside the usenet and also slowing down and glitching the game. Eventually The Boss makes it to the central kernel and Miller transfers his consciousness into the usenet as well. He takes his avatar form R4gN0r0K, a massive cyber dragon with a gigantic sword, to personally battle The Boss hand to hand. Miller claims there is no way he can lose as, "In this world, I am a god." After a climactic battle with The Boss, also in cyber dragon form after Kinzie hacks in and copies Millers avatar onto The Boss, Miller is defeated. Before The Boss can kill him Miller pleads for his life and bribes him with using his hacking skills to grant the Saints priority client status in either a weapons manufacturer or a car customization business. The Boss agrees and lets Miller live.
Disgraced, Miller quits The Syndicate and abandons The Deckers, moving back to Europe. Killbane allows him to leave without killing him in return for Miller originally backing him and serving faithfully after Loren's death, and even offers to act as a reference for him in his future employment hunts. A news report later reveals that Matt left the world of hacking, and that he always had an interest in post-modern art, hinting that that may be his future calling.
In the first mission of Saints Row IV, the Saints are pitted against Cyrus Temple once again only this time with Miller on their side. Since the events of Saints Row: The Third, Miller has become acquainted with an MI6 agent called Asha Odekar, and has become one himself. When it is seen that his plan has failed, Cyrus attacks the boss in 1v1 combat. Despite his military training and his previous STAG occupation, he is shot by the boss, yet launches the nuke. As the boss climbs the nuke in an attempt to disarm it, Matt expresses regret stating that if he knew that the boss was so willing to sacrifice themselves, he wouldn't have tried to kill them back in Saint's Row: The Third.
When The Zin attack The White House, Matt and Asha are present but are abducted after getting the President (the boss) to the Oval Office. When it is revealed that Zinyak has collected most of the characters in separate simulations, much to the already free Kinzie's dismay, it is decided that it is the Saint's best interests to save Miller first more-so than anyone else. This is due to his knowledge with virtual reality.
Miller's simulation takes the form of a text adventure, because Zinyak's simulations to encapsulate the Saints personal hell. Depicted in it is a battle in the ring between Matt and Killbane.
Once free of his simulation Miller proceeds to help the President regain the rest of the characters including Asha, but not before adding working weapons to the mech suit the President came to rescue him in. During the mission when the President goes into Johnny Gat's personal simulation, Matt feels the urge to tell them that rescuing Gat is probably not the best idea. The President then attacks Miller, punching him in the facial region many times before being pulled off. Kinzie then explains why it isn't the best idea and how she agrees with Matt. Before leaving, the President reaches for Matt to pick him up from the ground and apologise.
Matt can become Nytefall during his loyalty mission with the player in which he and the player undergo a Nyteblade fanfiction he wrote. When Zinyak intervenes and re-writes script, Matt is forced to fight against Nyteblade effectively killing both his story and his favourite character. This is rectified when he is given superpowers native to the simulation and the President gives him all rights to the Nyteblade franchise.
Once Zinyak is murdered by the President in front of the Zin empire, they are crowned victorious. Not soon after, in a club all the main characters are seen dancing, Matt included.
In Saints Row: Gat Out of Hell, Matt secures a Ouija board but fails to mention that it was once possession of Alastair Crowley. When he, Gat and the President play, the cursor piece becomes possessed spelling the name Jezebel. Suddenly, the President is pulled through a portal into hell resulting in Gat and Kinzie following them. Miller says initially that he thinks the duo following the Boss was an awful idea but his mind was quickly swayed by Johnny's reply stating that whilst he and the President were away, he would be in charge.
Just before the return of Kinzie to the ship, Matt can be seen ordering Shaundi to do as he wished. Shaudi the questions why Asha would ever sleep with him, only for Matt to reply saying he couldn't believe she didn't also.