Tobin Dax was a joined Trill first mentioned in the Star Trek: Deep Space Nine episode The Siege.
He was portrayed by Colm Meaney in the episode Facets as Chief O'Brien had embodied Tobin during Jadzia Dax's zhian'tara.
Tobin was born as Tobin Fendus in the 22nd century. Tobin grew up to be an engineer. He was a shy and introverted man. As a young man he entered the initiate program to be joined to a host, and was selected to be the second host for the Dax symbiont.
During the mid 22nd century he helped the United Earth Starfleet in its efforts to improve its technology to fight the Romulans during their war with humanity. Following the war he worked with the Federation Starfleet to help integrate the technology that the member races of the new nation had contributed. Feeling that Earth had the most versatile designs, he believed that while future Federation starships would incorporate technology from all the member worlds, they would essentially look like Earth ships.
While on the USS Pioneer, he stepped in as engineer when the previous engineer was disabled. He discovered the formulas that would allow human built Federation starships to use shields at higher warp speeds. Tobin remained on the Pioneer as ship's engineer. The ship's CMO Dr. Liao discovered his joined nature during this time, but she respected doctor-patient confidentiality and kept it a secret. She encouraged him to let the ship's CO Malcolm Reed know as well, which he soon did.
Tobin left the Pioneer in 2165, feeling that space service was too dangerous. He lived on Vulcan for a time as the guest of his friend Skon and his wife T'Rama while figuring out his next steps. While on Vulcan he helped defeat an attempt by V'Las to overthrow Vulcan's government, and saved the USS Endeavour as well as the Vulcan Space Central station when he discovered the Endeavour's warp core had been sabotaged.
Not too long afterwards Tobin would meet Skon and T'Rama's infant son Sarek at a reception Skon had. He decided to have his own family and had children, including a difficult child named Raifi.
As an old man Tobin was involved in a plasma research project.
Tobin would eventually pass on, and all of his memories and experiences while joined would live on in the subsequent hosts.
Dax would eventually pass to Curzon Dax, who would remark that Tobin had never been drunk once in his life, a situation he remedied during his zhian'tara by getting Tobin very drunk when Tobin was embodied by one of his friends.
Jadzia Dax would remember Tobin as someone who had no sex life, little imagination, but was a skilled engineer. She was able to use his knowledge of impulse engines to repair the engines of long disabled vessels. During Jadzia's zhian'tara Tobin was embodied by her friend Miles O'Brien. She had to gently remind Tobin not to bite his nails during the zhian'tara as it would damage Chief O'Brien's fingernails. Tobin was pleased to learn that Jadzia admired the work he did on Fermat's theorem, and that she continued his work.