Joy is scored out of a maximum possible 200 points. It accounts for 1/3 of overall Mood. Players can increase their Genopet's Joy score up to its maximum of 200 by playing Fetch with their pets using Toy items.
Playing Fetch is the player action which can increase Joy. Joy decays an equal amount every hour starting from the time of the last successful Playing action, equivalent to 30 points every 24 hours. When Joy is 0, a Genopet's Mood is effectively capped at 1/3 of its possible maximum.