"The son of the lord of Pharae Principality, Roy was studying in Ostia when the Kingdom of Bern invaded the League of Lycia. His father fell ill at this time, so Roy assumed leadership of Pharae's armies. After his fateful meeting with the Princess Guinevere, his destiny became inextricably linked with the fate of the entire continent."

In a nutshell, Roy is the son of a powerful ruler named Eliwood in The Kingdom of Lycia. When his father fell ill, Roy took over as the commander of the armies. He led his army across the continent, defeating the armies of Bern, until, finally, he met Princess Guinevere. Roy then took it upon himself to go and save not only his homeland, but the whole continent from this outside threat.
Roy probably joined the Stadium after seeing Prince Marth of Altea do the same. Roy is a flashy fighter, using his own sword, the Sword of Seals, to pull off amazingly powerful attacks, mostly fire-based. He's an average, 15 year old fighter, and acts with the same rebellious nature typical of kids his age. Unlike the other teens at the Stadium, Ness and Mike, however, Roy has true power, politically and physically, and he will go out of his way to let you know it.
In actuality, there have been a few cases where Roy has powergamed his way to victory. He defeated anyone who challenged him, even forcing Killer to leave the Stadium. It wasn't until the brave Young Link took Roy on that the "Fire Emblem Fighter Saga" was put to rest. The Stadium is currently keeping a close eye on Roy, trying to estimate his next move, as well as his motives for challenging the fighters in such a manner.