Sigismund Dijkstra is the head of Redanian Intelligence, or secret service. He is a physically imposing man; nearly seven feet tall and bald, with a rather noticeable stomach and is said to give the impression of a scrubbed pig. He also doesn't dress the least bit like a spy, preferring bright colours and flamboyant dress. He is an exceptionally clever man and is said to be very influential in Redania. There is a considerable description of Dijkstra in the novel Blood of Elves where he is first introduced into the saga.
|“||Dijkstra, head of King Vizimir of Redania's secret service, did not resemble a spy. He was far from the stereotype which dictated that a spy should be short, thin, rat-like, and have piercing eyes forever casting furtive glances from beneath a black hood. Dijkstra, as Dandilion knew, never wore hoods and had a decided preference for bright coloured clothing. He was almost seven foot tall and probably only weighed a little under two quintals. When he crossed his arms over his chest - which he did with habitual pleasure - it looked as if two cachalots had prostrated themselves over a whale. As far as his features, hair colour and complexion were concerned, he looked like a freshly scrubbed pig. Dandilion knew very people whose appearence was as deceptive as Dijkstra's - because this porky giant who gave the impression of being a sleepy, sluggish moron, possessed an exceptionally keen mind. And considerable authority. A popular saying at King Vizimir's court held that if Dijkstra states it is noon yet darkness reigns all around, it is time to start worrying about the fate of the sun.||”|
— pg. 196-197, Το Αίμα των Ξωτικών (UK edition)
During the Second Nilfgaard War, King Vizimir of Redania was assassinated. During the interregnum, the country was ruled by a Regency Council headed by Dijkstra and Philippa Eilhart. This made him basically the most powerful man in the kingdom.
In the aftermath of the Treaty of Cintra, Dijkstra tells Philippa Eilhart that he is hot on the trail of the conspirators that killed king Vizimir. When an assassination attempt is made on his own life, he realizes that he may have confided in the wrong person and flees Redania. At the foot of the Elskerdeg Pass, he meets with Isengrim Faoiltiarna and Boreas Mun, both fugitives for their own reasons. The three of them form an unlikely travelling party, heading East, towards Zerrikania or even Haakland, presumably leaving the territories of the Nilfgaard Empire and the Northern Kingdoms for good. To his companions he introduces himself as "Sigi Reuven".