Callisto is a huge bear that resides south of the Demonic Ruins. The best way of fighting him is using Protect from Melee and standing within melee distance because his special attack, which cannot be avoided, can deal up to 60 damage and will be used more often if you are not within melee distance. Callisto also uses an attack that sends you sprawling across the ground much like The Giant Roc from My Arm's Big Adventure. The knockback does 3 damage every time and you are unable to eat while you are knocked back. The knockback has an area of effect, so nearby players will also be sent back along with his current target if they are next to him. Callisto can also heal himself; a red blast will appear under him, and the next incoming attack that does damage will heal Callisto a small percentage of what that hit dealt.
Callisto is named after one of the Greek god Zeus' many lovers, who was turned into the constellation Ursa Major.
Before mid-March 2014, players could safespot Callisto using the Trident of the seas with the "Longrange" option behind a cluster of skeletons. This has since been patched and the cluster has been removed.
Callisto is the name of one of Jupiter's moons.
His examine text is a reference to Goldilocks and the Three Bears.