Cerberus

"Infernal watch-dog Cerberus regards the Underworld Prince with mixed emotions, from purest joy to deepest melancholy."

Cerberus is a massive three-headed dog charged with guarding the gates of the Underworld to prevent the dead from escaping. He is a magnificent beast, companion of Hades and Zagreus.

When Zagreus left the House for the first time, Cerberus threw a tantrum that wrecked the House's lounge area.Once Zagreus reaches the Temple of Styx, Cerberus guards its exit. Zagreus must find a Satyr Sack, Cerberus's favourite snack, as a bribe to leave the area.

Affinity
If given Nectar, Cerberus gives Zagreus the Old Spiked Collar.

Zagreus can pet him at any time in the House.

Cerberus' affinity gauge maxes out at 9 hearts.

Favor
To complete Cerberus' favor, he must be pet at least 20 times. (Pets given before the final favor lock was added to the game are not counted, so fortunately you must pet him another 20 times)

Petting him 10 times earns the Three-Headed Boy achievement, which is an indicator that you're at least halfway there.