Isaac is a young man who had awoken at Aperture Science, after Chell has left.
As a young boy, Isaac always had a fascination with science. When he was 18, he heard about the testing at Aperture Laboratories and instantly volunteered. He was kept in stasis for years, until a raid on Aperture Sciences, by Civil Protection. Shortly after, the Xen aliens arrive, shutting down GLaDOS, killing some of the Civil Protection, and overtaking Aperture Science. Isaac woke up from stasis, to be greeted by bloody messages on the walls, scattered dead bodies, and Xen aliens. Armed with a Portal Gun and pistol, Isaac fought through the aliens. Later, he discovered GLaDOS, and reactivated her. GLaDOS released neurotoxin on the Xen aliens, but told Isaac that he still needs to make it through the rest of the chambers. As Isaac traversed further, he made it out finally, keeping in contact with GLaDOS. He later moved to City 17, later adjusting to his new surroundings.
Isaac has black hair, often wears jeans, not very muscular, and wears a shirt that says "Do it for science".