Origin of name
In selecting Simon Illyan's name, Bujold paid homage to two 1960s television characters. His last name is from Illya Kuryakin, a character in The Man from U.N.C.L.E., and his first from Simon Butcher, a character in The Wackiest Ship in the Army.
Illyan begins as an operative of Barrayaran Imperial Security Service - imperial intelligence and counter-intelligence agency. He has had a bioelectronic device implanted in his head: an eidetic memory chip that records everything he sees and hears and allows him to recall that information instantly. This was done on a whim by Barrayaran Emperor Ezar, after he learned that the technology had been recently invented on the planet Illyrica. Since most such implants produced schizophrenia or worse in their hosts, it is clear that Ezar regarded Illyan as expendable.
Illyan first appears in Shards of Honor as a lieutenant sent to keep an eye on Aral Vorkosigan, one of the story's main characters, on his mission to conquer the planet Escobar. Eventually becoming Vorkosigan's right-hand man, in Barrayar he achieved the station of BISS's head, following the death of previous incumbent, Captain Negri. Since then in-universe he is referred to as Captain Illyan.
With Barrayar in relative peace, Illyan was able to move ImpSec towards a more benign role on Barrayar while extending its reach to imperial conquered system of Komarr and beyond, though he had to deal with more than a few harrowing domestic events, such as the Yarrow Incident, in which "ImpSec" averted an assassination plot on Emperor Gregor Vorbarra.
His biochip-aided memory added to his mystique as an administrator with ferocious attention to detail. Series main character, Miles Vorkosigan, who worked under Illyan as a special agent, surmised that Illyan's successful integration with the chip was due to an innate ability to suspend belief or disbelief in multiple, conflicting sets of facts—an ability also very useful to a spymaster. Illyan started to appear more frequently in the stories after The Vor Game, when series main character, Miles Vorkosigan, became his agent in "ImpSec" due to Miles' inability to work with a strict chain of command. (The initial reasoning was to bury Miles deep within ImpSec where he couldn't cause trouble, but Miles startling success on the most difficult cases made him Illyan's top agent, especially in his position as "Admiral Miles Naismith", head of the Dendarii Free Mercanaries, a Barrayaran Crown Troop covertly posing as a mercenary fleet). Despite his unnervingly roundabout and inventive methods that would drive Illyan to distraction, Vorkosigan became Captain's greatest annoyance and most powerful asset.
Illyan's career ended in Memory when his memory chip broke down, causing him to relive events from his past at random. Although he made a good recovery, he had to retire; after 40 years of relying on perfect bio/electronic recall, his short-term memory was almost nonexistent. First Miles, and then Miles' aunt, Lady Alys Vorpatril, cared for him as he slowly learned to start remembering things for himself again.
As for A Civil Campaign, Illyan lives in a nondescript flat, not far from Alys (whom he is now dating), and maintains an informal relationship with ImpSec. One of his most important roles now is being a go-between for ImpSec and certain covert high-Vor (aristocratic) informers.
In appearance, Illyan is slight and extremely unimpressive. As ImpSec chief, he amplified this lack of presence to the point of virtual invisibility, the better to observe others. In his own domain, however, his air of blandness makes him seem all the more menacing.