Ari is Jeb's seven-year-old son. At age three, scientists turned him into an Eraser, which is a fearsome and power-hungry enemy of the flock. Ari had blond hair until he faced Nudge and Fang in

The Angel Experiment (Nudge spray-painted Ari's hair green).

In the first book, Ari and several unnamed Erasers capture Angel. This leads to the flock going to the School to reattain her, and this is where Max discovers that she is supposed to save the world.

In book two, Ari teams with a clone of Max, whom is referred to as Max II. Max and Max II battle at the end of School's Out--Forever after the clone takes Max's role as the leader of the flock.

In the third book, Max invites Ari to join the flock after all of the other Erasers are eliminated. Fang, who is still against the idea, splits the flock into two groups: Max's group includes herself, Nudge, Angel, Ari, and Total; Fang's group includes himself, Iggy, and Gazzy. Max's mini-flock roams France, Britain, and Germany, where they meet the Director. Ari and Max find out from Jeb that they are half-siblings. Max suddenly doesn't despise Ari anymore, and then he expires in her arms during a battle.

Book four begins with Ari's funeral. Jeb and the flock are all present.