Brother Adrian is an unseen character in Grand Theft Auto V.
Adrian is the leader of the Children Of The Mountain movement.
At some point of the storyline, he will send various text messages to Franklin Clinton and will try to convince him to join by giving him their website address and a form to complete. Franklin can do this and will receive another text from him which will give Franklin a password to access the stage two part of the movement. After Franklin completes all the steps on the website, he will receive a Children of the Mountain t-shirt in his wardrobe and Brother Adrian will tell him that he will keep in touch with him.