Basically, Sean is a kid genius. Able to run any kind of program from his simple laptop (a antique where he's from.)
In the manga, Sean was over taken by a dark force that compelled his dark side, making him nothing more than a pawn in a bigger game. He started out as a mole for the Officials while he was running Grave.
In the video game, Sean's parents died in a plane crash when his was very young. He was forced to move with abusive relatives. Since then, he surf the Net in order to escape his troubles. It was then that he encountered a Navi that promised him the world. Sean then ran away and, under a hologram disguise, became a ringleader for Grave, an allience of people who hated the world too. But each member in the organiaztion was brought down by Lan Hikari and Megaman.EXE. One day, Lan and Megaman, along with their friends, face off againist Sean during his final phase of his master plan: To create Bass.EXE, the ultimate NetNavi to take over the world. But the program went haywire, turning Sean's progam wild and uncontrollable, but Lan and Megaman saved the day. Lan then read Sean's journal, learning of the Sean's past. With his plans foiled, Sean wished to died, cause he didn't have anything to live for. Lan gave Sean his hand and said.
"Once you atoned for what you've done, I promise to be your first friend."
With that, Sean just cried, knowing that he's longer have to be alone. Since then, Sean has travel the world and trying to make things right.