Jeb is a scientist who works for the School, and is Ari's father. When Max was ten, he broke the flock out of their cages and brought them to live with him in the vacant mountains of Colorado. When Max was twelve, Jeb disappeared, which left the flock to fend for themselves. Everyone assumed that Jeb was dead, and never talked about it.
In The Angel Experiment, when Angel is kidnapped by Erasers and brought back to the School, the rest of the flock goes to save her. They find that Jeb is still alive and working for the evil side (often referred to as Them in later books). Jeb tells Max that it is her destiny to save the world, but she is doubtful.
In School's Out--Forever, Jeb appears several times. He was once at Anne's home, when the flock found that she had some history with Jeb. Jeb again came at the end of the book, when Max fought her clone, Max II. Jeb said that there could "only be one Max".
In Saving the World and Other Extreme Sports, Max and her mini-flock (Nudge, Ari, Total, and Angel) are held hostage in a dungeon by the Director, who lies to Max by insisting that she is her mother. Later, Jeb appears and informs Max that he is her father, and that Max's biological mother is Dr. Valencia Martinez. This makes Ari and Max half-siblings, which Max is (at first) uncomfortable with.
In The Final Warning, there are few mentionings of Jeb; one of the only times where he appeared was in the beginning of the book. This was during Ari's funeral, and for some time afterwards.
In Fang, Jeb begins to side with another scientist named Dr. Hans Gunther-Hagen.