Ethan Frye is the father and teacher of Jacob and Evie Frye. He dies of pleurisy in January 1868, just before Assassin's Creed Syndicate begins, so he never appears in a memory, but his lessons and his books shape the twins' choices throughout the game.
Ethan's central rule, never let personal feelings compromise the mission, is a running thread. Evie holds to it and hesitates over her feelings for Henry; Jacob resents it and sets out to prove himself without his father holding him back. A book from Ethan's study is what eventually explains the Shroud of Eden.
During Family Politics Duleep Singh tells Evie, on Henry's information, how deeply Cecily's death affected Ethan, and suggests grief made him say things to protect his children. Evie comes to think her father was wrong about some things, which lets her accept Henry.
On his deathbed Ethan kept the twins at a distance so as not to burden them and asked George to complete their training. Only George, Jacob and Evie attended his quiet funeral in Crawley, in keeping with Assassin custom.
Cecily Frye
Cecily Frye is the mother of Jacob and Evie Frye. She dies after giving birth to them in 1847, so Assassin's Creed Syndicate only refers to her, chiefly when Duleep Singh explains to Evie how her death changed Ethan Frye.
Duleep Singh
Duleep Singh, the last Maharaja of the Sikh Empire, is Henry Green's great-uncle. He helps Evie find the Buckingham Palace plans in Family Politics, and is the central figure of The Last Maharaja missions, where the twins help him fight the British India Company.
William Gladstone
William Gladstone is Disraeli's rival and the Templars' preferred Prime Minister. In Double Trouble Jacob steals his and his wife's invitations to the Buckingham Palace ball, along with their carriage.