This character becomes the target of brainwashing.

Brainwashing refers to a process in which a group or individual "systematically uses unethically manipulative methods to persuade others to conform to the wishes of the manipulator(s), often to the detriment of the person being manipulated". The term has been applied to any tactic, psychological or otherwise, which can be seen as subverting an individual's sense of control over their own thinking, behavior, emotions or decision making.