Andrew Harvey is an agent with MI-5.
When we first meet Harvey, he is quite near 40 years of age (he might be approaching it or having just seen it go by). He has been an operative with this British internal security organization for many years and has grown to be one of their most trusted people. After so long in the service, he has acquired a refined sense of honor as well as a fairly reliable BS meter, not to mention just enough diplomatic skill to not get in trouble when that meter pegs too far up.
We encounter Harvey as part of the activities in the "universe" of , a man who fought back against injustice and paid a huge price for doing so. It was Harvey who was one of the key men tasked with stopping Gray from what really amounted to embarrassing the government by taking on the apparently invulnerable people and winning. Harvey walked a fine line between stopping Gray and helping him, enough so a little of both got done.
As time, and the series, continue, Harvey will have other missions which will take his attention and his savvy though once or twice he found being able to call on Gray and Gray's associates in the private security business they set up a very useful asset.
Another person he has at his back and at his side is Sarah, the very experienced former MI-6 who is forced to work with Harvey on an early mission and with whom he would start a deep and caring relationship, despite neither liking the other at first.
So between Harvey's impressive skill set and his girl friend's as well as his now closeness to Gray and company, this makes for a solid team.