Stephen Bellecroix and Sheila Roath are agents for British Intelligence.
The exact department is not mentioned directly but MI-5 is alluded to and the work that these two agents perform would fall into their mandate. The two are not partners per se but are friends and are assigned to the same case.
Bellecroix is described as a half-caste, both of a white Welsh mother and a black West Indies man, raised in the rough and tumble region of Cardiff called Butetown by some and Tiger Bay by others. where he learned to take care of himself in a fight. His upbringing was definitely a lower class one and his parents still live in the ramshackle house he grew up in. Somehow he managed to break free from that poverty, probably via scholarships, and attend University. He was even able to continue his education in America from which he acquired a different accent from his original one. He attended Berkeley in California before taking part in student protests in the 60s got him sent home.
Sheila Roath is daughter of a notorious viscount, a man who had served for years in the Foreign Office before deciding he had risen as high as he likely would and saw no reason to continue. He was renown for his wild parties and bohemian ways, which is perhaps where the young Roath learned her independence which she shows without hesitation. Roath likes men, a lot, and she likes to change companions on a fairly regular basis. As the series starts, she is more of a Researcher than a field agent but that changes soon into the first book.
The two have a relationship with each other that, as per Roath's desire to change on occasion, is best defined as periodic. Roath at one point thought that a permanent basis might be in order until she visited Bellecroix's parental home and realized that their two worlds would never mesh, something Bellecroix knew from the beginning but still hoped might be different.
A third agent named Hugh Liversidge has an equal role in the first book's adventure, and an equal role in Roath's attention, but only Bellecroix and Roath are in the second.
An interesting aspect of the Bellecroix and Roath stories is the look behind the curtain at the operations and politics that play such an important part in the proceedings. Who is up at a certain time naturally decides which plan is implemented, regardless sometimes of the merit of any of the plans. And placement of people in the hierarchy of power is as much about the ability to assign blame when things go wrong as it is to reward when things go right. This office politicking is nothing new; it is just very well displayed in this series.