Sigma (シグマ, Shiguma?, Σ) is the "True" Master of Watcher in the False Holy Grail War of Fate/strange fake. Despite having summoned Watcher, the shadows of Watcher claim that it is not meant to be a participant of the Holy Grail War, but rather a guide for Sigma himself to become the True Lancer (真ランサー, ?) Servant of the War "while living."
Sigma apparently has some knowledge with film related details such as how he tries to hide Watcher's identity, informing others that Watcher is Charlie Chaplin, or how, when Watcher asked the young Magus if he knows about King Arthur's Holy Grail quest, Sigma responded that it's a Monty Python comedy which causes Watcher to falls silent.
Sigma is a mercenary hired by Francesca.
He was first mentioned in Fate/Zero novel. He was indirectly mentioned when Maiya confesses to Irisviel von Einzbern about being a mother. Irisviel encourages Maiya to look for her son after the Fourth Holy Grail War.