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.
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".