Kathy and Ted rescued and adopted Isaac from a war-torn part of the world when he was just a baby. He is meant to represent a pure, child-like innocence unspoilt by propaganda and trauma. Everyone has an inner child buried under the baggage accumulated as an adult. Isaac serves to remind Joel that that innocent child exists within himself.

Isaac looks up to Joel and admires his big brother a lot, even when Joel doesn't feel like he deserves it. Hyperactive and has a powerful imagination. Obsessed with dinosaurs and outer space, often making stuff up about them and telling Joel about it like its fact.

An incident causes him to gain an awareness of death that threatens his carefree outlook.