A clone of Ryan Stevenson, Matt is one of 25 'copies' grown in an Alterran incubation chamber, having the same genetic structure and memories as Ryan, but a different core energy. Similar in nature to twins, clones are different people in the same 'body,' though over time that body will alter to match the 'person' inside, which is why some twins appear the same 30 years down the line and some will vary drastically in appearance.
Matt, along with his brothers, each assigned themselves to a needed task in the initial rebuilding phase of the rebirth of the Alterran civilization to lessen the load that Ryan had been carrying mostly on his own, choosing to take over primary medical duties within Atlantis, though his knowledge of the subject was identical to each of the other Stevensons.
Most of his initial duties were patching up injuries to the allied Lantean and Human populations, though his most notorious 'patient' was Ryan himself, suffering from multiple self inflicted injuries, most noteably burning off the majority of his skin during an accidental bio-energy overload while in his quarters. Matt initially regrew Ryan's skin using his own regenerative healing ability, then took his brother down to Atlantis's medical facility for analysis of the mysterious 'malfunction.'
Along with Andara, Matt exhaustively studied all the Lantean records left behind on the Wraith and redid their analysis of the species, spotting several inconsistencies and blatant errors they'd made. With a fresh look at the data, he was able to make several valuable insights, noting the distinct danger of ever allowing a Wraith to feed on an Alterran, for they would adapt to the new genetic structure and grow insanely more powerful, as well as sucking some valuable nuggets of knowledge out of the victim in the process.
Both he and Ryan agreed there was little chance of this happening, given how small the current Alterran population was, but it meant that the Wraith were far more dangerous than the Lanteans had ever realized.
As more Lantean medical personnel came into the fold, noteably Carson Beckett, Matt's daily work load dropped dramatically, freeing up time for him to deal with other non-medical matters. Given that he, like all of his clone brothers, has the full knowledge of the Repository crammed into their minds, Matt is ideally suited to deal with just about everything that goes wrong around Atlantis, though more often than not he is called to situations outside the city, and even occasionally outside the galaxy, that the others are too busy to deal with.
Matt was the closest available Alterra at the time of the attack, having been in Atlantis on medical duty, and immediately traveled to the planet via stargate to assume military command and man the planet's heavy weapons. Despite racking up a considerable kill count, the cities had to be evacuated and destroyed in order to deny them to the Wraith/Romulans who circumvented the defense shields by burrowing hidden tunnels underneath the shield perimeter and invading the sanctuary cities from underneath.