Jayne Robinson is a freelance agent.
When we first meet her in the initial recorded adventure of CIA operative , she is an agent with MI6 and she will remain with them for quite some time and a good number of missions before going off on her own. Once she becomes a free agent, she will continue her association with that organization while also going on a good number of assignments for the CIA. Like her friend, Johnson, she might be considered a private investigator of sorts but considering the work she primarily does for both MI6 and the CIA, freelance sounds more accurate.
That first contact revealed she had been quite active in late 1980s in the highly dangerous regions of northern Pakistan and, despite the attitude of most men in that area towards women, has established some very impressive sources of intelligence. Some time later we learn she had been transferred to the much nicer region of Buenos Aires where she again was able to set up a considerable number of contacts about the Argentine regimes. This was followed by a few years in the ever-changing political world of the Balkans during the early 2000s, working especially hard in the prosecution of war criminals. After her cover was blown by a foolish reporter, she had to leave that region; her new assignment put her in Northern Ireland for a while.
As mentioned, she plays a major role in all the earlier Joe Johnson adventures, starting with a mission in Afghanistan taking place in the late 80s. Since we can postulate she attended college before her MI6 service and she had already been in country for a couple of years, we might reasonably estimate her to be in her late 20s when we first meet. We are then able to catch up with on a spotted basis over the couple of decades up to the point where she decides to leave the SIS after 26 years and go out on her own.
Jayne Robinson is described back in the first meet as a slim, tanned woman with short dark hair and a white smile. When she spoke, it was with a "whiskey-low voice" that seemed to be especially seductive to most men. Even with the many years of action she had accumulated before she can still spark interest when she enters the room and her devotion to maintaining her physical conditioning has come in handy more than once when she has had to vacate that room in a hurry. Over the years she will mature but based on Joe Johnson's reaction throughout all that time, her beauty and grace has not diminished.