Kallisto was a Hunter of Artemis and the former best friend of the goddess. However, Zeus was attracted Kallisto and in order to get close to her, he disguised himself so as to look like his daughter. While Kallisto was separated from the group, he seduced her in this form. Kallisto would become pregnant during this encounter and tried to keep it from showing.
Eventually, Artemis and the other Hunters discovered her condition while bathing and the goddess was furious. Though Artemis said to Kallisto that it wasn't her fault, she shouldn't have lied to her. Instead of letting her go out of the Hunt in peace, Artemis told her former best friend to run, for she and the other Hunters would hunt her down if they came across her again.
Kallisto (as a bear) eventually gave birth to a human son by the name of Arkas, but she was later tragically killed by hunters (though it remains unclear whether Kallisto's killers were the Hunters of Artemis). Out of pity and possibly guilt for his own actions, Zeus transformed Kallisto into a constellation; the constellation which would become known as Ursa Major. Arkas almost shot his own mother with an arrow, but before he could Zeus turned him into a bear and set him into the stars, Arkas became Ursa Minor.
Zoë Nightshade uses Ursa Major to lead everyone west through the Junkyard of the Gods. When Percy Jackson referred to it as the bear, Zoë called it a "worthy opponent," suggesting she had fought it at some point in her past.
- If Zoë was indeed the one who hunted the bear that became Ursa Major, that would mean Kallisto was in fact killed by a Hunter of Artemis.
- Kallisto and Arkas can also be spelled with c's instead of k's.