Thomas Riker was a duplicate of William Riker created by a transporter accident, who later joined the Maquis and stole the USS Defiant.
Thomas Riker was created when William Riker beamed up from the planet Nervala IV during an evacuation. A rare atmospheric effect split the transporter beam, so one Riker returned to his ship while an identical copy was reflected back to the surface. Believing he had been abandoned, the copy lived alone at the outpost for eight years.
In 2369 the Enterprise-D found him. The two men clashed, and the duplicate rekindled his feelings for Deanna Troi. He chose to use his middle name, Thomas, and accepted a posting on the USS Gandhi. He later left Starfleet and joined the Maquis. In Deep Space Nine he posed as William, stole the USS Defiant and attacked a hidden Obsidian Order base. Captured by the Cardassians, he was sentenced to a labor camp.
Thomas Riker gave Jonathan Frakes a chance to play a darker version of Will, showing how different experiences could shape the same man.