Ulysses was a Goa'uld in an Unas host who once served an enemy of Ares before Ares managed to destroy him. When this happened, Ulysses was taken into Ares' dungeon and tortured for years to make an example for any others who would defy Ares. It was in that dungeon that he learned tolerance, acceptance, and even compassion from a fellow prisoner, becoming a Tok'ra in spirit if not in name.
Sometime before the fall of Greece, Ulysses was transported through the Stargate to the Goa'uld graveyard world of Kar'tek where he made friends with the native Volca even using some form of technology to give them a natural ability to phase through solid objects; this technology was based on an electronic device which seemed similar to that of the Tollans though less powerful. Ulysses kept himself hidden within the rock for many years before SG-1 came to the planet.
Etahy took Daniel Jackson to see Ulysses after the rest of SG-1 had been captured to seek his help. Although Ulysses did not initially seem happy to see Daniel, after questioning him on the state of Earth he seemd to warm up to him slightly, going so far as to tell him of his past. He gave Jackson the phase shift device he had and informed him that it would only last for five minutes, and demanding that he not let it fall into Goa'uld hands lest they reverse-engineered it.
Shortly afterwards, Daniel Jackson and Etahy managed to use their phase-shifting ability to free SG-1 and Etahy's family. During the course of these events, they believed they had killed Mars—though he actually survived—and so afterwards planned to rebuild their government. On Daniel Jackson's suggestion, Etahy led Mars' former Jaffa to Ulysses so that he could help them in this regard. (SG1: "Fall of Rome 1")
- Ulysses on the Wikipedia