Thomas Riker was a transporter duplicate of William Riker, created in a transporter accident. He eventually joined the Maquis, a rebel group, and was later captured by the Cardassians.
In 2361, Lieutenant William Riker was serving aboard the USS Potemkin when a transporter accident occurred during an evacuation mission on Nervala IV. The transporter beam was reflected back to the surface by the planet’s distortion field, creating two identical Rikers, each with the same memories and experiences up to that point. The duplicate Riker, who would later be known as Thomas, was stranded on the planet for eight years, as the Potemkin crew believed there was only one Riker and had rescued him.
Thomas Riker was discovered in 2369 by the crew of the USS Enterprise-D. At that time, he decided to use his middle name to differentiate himself from William Riker. Thomas struggled to find his place, feeling overshadowed by the accomplishments of his counterpart. He eventually joined the Maquis, a group of freedom fighters (or terrorists, from the perspective of the Cardassians and the Federation) who opposed the Cardassian occupation of what they considered to be Federation territory.
His activities with the Maquis led to his capture by the Cardassians in 2371, as depicted in the Star Trek: Deep Space Nine episode “Defiant.” Thomas Riker impersonated William Riker to commandeer the USS Defiant in a mission against the Cardassians. After his capture, he was sentenced to life in a Cardassian labor camp. His fate beyond this point has not been explored in the canonical Star Trek television series or movies, leaving his ultimate destiny unknown. However, his story has been further developed in non-canon Star Trek novels and other media.