The CIA agent who found Charles Xavier before anyone else did, and who the films keep bringing back with a different face and no memory of him.
Moira MacTaggert is a CIA agent, a doctor and an ally to the X-Men, and she has been played by two people. Olivia Williams is the first, in X-Men: The Last Stand, and Rose Byrne is the second in every appearance since. The order the films came out in is the reverse of the order she lives them.
In First Class, set in 1962, she is an American CIA agent who infiltrates the Hellfire Club and begins working with a young Charles Xavier to stop them. She is initiated into Division X, helps Charles and Erik train the first class, and is on the beach in Cuba for the final battle, where she fires at Magneto and one of the deflected shots goes into Xavier’s spine. She becomes his caretaker until he wipes her memory, and leaves the CIA to become a doctor, which is what she is in The Last Stand: teaching students at the Institute remotely from her base on Muir Island, where she cares for a brain-dead patient the source identifies as Xavier’s brother.
Minor spoiler
After Xavier’s death his consciousness enters that patient’s body and greets her. It is the scene after the credits, and it is why he is alive in the films that follow.
Apocalypse is the revised timeline, so none of that has to hold. Moira is still at the CIA, has had a son, and has been married and divorced. Investigating mutant cults in Cairo she discovers Apocalypse’s awakening. After the mansion is destroyed she is taken by Colonel Stryker and interrogated until the student X-Men free them, and after the final battle Xavier restores her memories. She has Stryker arrested for treason at the end of the film.
Show detailsThe reader's spelling
The list that put her here spelled the name McTaggart. Wikipedia’s list of X-Men film characters has MacTaggert, and that is what the entry uses.