Sullivan was the commander of a F.I.R.M. Assault team. He led the team to storm the town hall of Beaudy which Billy Fargo had converted into a secret missile launch facility.
In Desperate Monday (episode), Sullivan shows up again in an uncredited role. His name is not actually given so it is just assumed to be Sullivan. It could just as well have been Price. Here he is asked by Archangel to deploy his Zebra squad and make a risk assessment of a hostage rescue situation.
In other episodes, the F.I.R.M. assault team (not always called the Zebra squad) had leaders such as Price and Hansen.