Tate is a mechanic droid similar to T3-M4 from Star Wars Knights of the Old Republic I & II.
The story takes place during the New Sith Wars and at this time while they may be referred to as astromech droids this style of droid was actually just a mechanic droid. It would be thousands of years before the first series of R astromech droids would be released.
Tate slipped aboard a shipment of protocol and training droids destined for Dantooine near the beginning of the New Sith Wars. When he arrived he attached himself to a Padawan and eventually saved that Padawan from succumbing to the dark side by shocking a Sith with his arc welder. The Sith had been talking about the insignificance of droids and how a droid could never harm him when the electric jolt caused him to fall backwards into a pool of lava. The Padawan realized the Sith was saying anything he could to turn her and returned to Dantooine where she and Tate were given rewards and prestige. Her reward and prestige was becoming a Jedi Knight while Tate was given free reign of the temple and allowed to go on missions that he enjoyed.