A dying genius who considered himself Data's grandfather and secretly moved his own mind into Data's body.
Dr. Ira Graves was an eminent cyberneticist who had been a teacher of Noonien Soong, which led him to call Data his grandson. In The Schizoid Man the Enterprise-D answered a distress call from his remote research world, where he was dying, along with his assistant Kareen Brianon.
Before his death Graves transferred his consciousness into Data, leaving his own body lifeless. Data began behaving erratically, showing jealousy and arrogance, until Picard confronted him. Graves finally released Data and placed his knowledge into the ship's computer, without his personality.