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.