Male Trill who was the second host to the Dax symbiont. Tobin Dax was a shy, nervous fellow, a nail-biter who was socially inept and constantly apologetic. But when it came to intellectual pursuits, he excelled in several fields. He knew phase coil inverters like no one else, and prior to his death he was working on proving Fermat's Last Theorem, using the most original approach to the proof in 300 years. He tried his hand with botany, but despite being a vegetarian, he didn't have much success there. He also dabbled in sleight-of-hand magic.

Tobin had a child named Raifi who put him "through hell" (according to Jadzia Dax), but that could be due to the fact that he could never bring himself to discipline his children no matter what they did. Once while on Vulcan, Tobin Dax met exiled Cardassian poet Iloja of Prim, giving him an interest in Cardassian literature. Iloja remained Dax's favorite Cardassian writer in subsequent lives.

The exact time of Tobin Dax's joined life is unknown, but it was probably in the late 21st or early 22nd century.