Little is known about Tobias' past, but at some point he managed to capture the Legendary Pokemon Darkrai and Latios. According to rumour, Tobias defeated all of Sinnoh's Gym Leaders using only Darkrai.
During the Sinnoh League, Tobias was a mysterious trainer, not really making his appearance known. No one was able to defeat his Darkrai in the entire tournament except for Ash but Ash tied with Latios. Tobias won the Sinnoh League with remarkable ease, though he said to Ash that he has the making's to become a Pokémon master. Although what Tobias did after the tournament has not been revealed it is assumed that he went on to battle the Sinnoh Elite 4, since he was allowed to do so as the Champion of the Lily of the Valley Conference.
Darkrai is the main Pokémon of Tobias's team. It is unknown how Tobias managed to catch the legendary Pokemon but Darkrai was a devastating fighter whose only known loss was to Ash's Sceptile.