Jax was a young male Mirialan who was kept as a prisoner in the vault of Tantiss Base, where he was designated SP-32. He, along with two other children named Eva and Sami, were imprisoned due to their high M-count. At one point, he tried and failed to escape, resulting in him getting stunned and being put in isolation for two rotations. He and the other children would eventually escape Tantiss with the aid of Doctor Karr and the Bad Batch.