SP-99 was a protocol droid working for the reconstituted Sith Empire during the Great Galactic War.
After the Sith forced the Republic to sign the Treaty of Coruscant, Jedi Master Dar'Nala and Jedi Knight Satele Shan were dispatched to the Outer Rim Territories to oversee the withdrawal of Republic troops from worlds ceded to the Empire in the treaty. SP-99 was assigned to monitor the Republic and Jedi activities to ensure full compliance with the treaty.
SP-99 was standing just outside the cabin occupied by Master Dar'Nala and Jedi Shan as they were discussing their dissatisfaction with their current assignment and with the treaty in general. Dar'Nala sensed the droid and threatened to melt his memory core with her lightsaber. SP-99 reminded her that she would be in violation of treaty were she to perform such an act. After Dar'Nala stormed off, Shan assured the droid that the problem was not him, but his masters.
While passing through the Kuat sector of the Core Worlds, their transport came under attack from a Sith warship. After Satele killed two Imperial commandos, he reminded her that the act was in violation of the treaty. She then sarcastically complimented his power of observation. After Dar'Nala told Satele to flee after she was captured and to warn the Republic of the Sith's treachery, the pair made their way to an escape pod.
Espee discusses the situation with Shan, Tavus and Din
She then told him that he could stay there with "His friends." He refused saying "That is not my assignment." When they were on board the escape pod Satele commented that she would save her master. He stated that that was highly unlikely and it would take approximately 500,000 years for the escape pod to be located in that sector. Satele sarcastically replied that would teach her patience.
The escape pod was rescued from slavers by a Republic shuttle that was on route to Balmorra. The Jedi Master Orgus Din and Lieutenant Harron Tavus brought Satele and SP-99 aboard. When Satele informed them of the Sith's treachery, SP-99 tried to discredit her by saying she had not witnessed the attack from the beginning. Orgus Din, however, disregarded his interruption and resolved to inform Master Zym of the Sith's actions.
Later on Balmorra, he offered to accompany Shan and Tavus in continuing their unfulfilled mission of overseeing the withdrawal of Republic forces from Dantooine under the terms of the Treaty of Coruscant. Following the attack on the Republic envoy Dar'Nala, the Republic forces on Dantooine had refused to leave with their commander and were still engaged against Sith forces, in violation of the Treaty. SP-99 reiterated that his assignment of monitoring the Jedi and the Republic was still unchanged.
SP-99 is destroyed by Tavus after eavesdropping on him
After Tavus caught SP-99 spying on his conversation with Fortris Gall, he told him that it was his duty to do so to ensure compliance with the treaty. Tavus shot and destroyed the droid, telling him he wasn't accepting that as an excuse anymore.
Constructed by the Sith Empire during the Great Galactic War, SP-99 was programmed to be completely loyal to his Sith Masters. Though he was able to interact at a personal level with Republic and Jedi personnel, he was viewed with suspicion and even disliked by some of them due to his Sith origins. Despite fighting on opposing sides, SP-99 was still able to form a sympathetic relationship with the Jedi Knight Satele Shan who had a more conciliatory approach than her Master Dar'Nala. Still his allegiance and defense of the actions of his Sith masters placed him at odds with other Republic and Jedi personnel.