"Happiness" is an in-game mechanic that indicates how friendly the Pokémon is to the owner, from a scale of 0 to 255. You can check a Pokémon's Happiness in its summary, on the party screen, or when you hover over it when viewing your Fields.
A few Pokémon require a certain amount of Happiness to evolve. There are currently 15 Pokémon that evolve from having high Happiness. In order to evolve, the Pokémon must have a Happiness of 220 or more.
Each Pokémon has a type which affects what fields the Pokémon likes. Pokémon like the fields that are super-effective against them, and their own type, while disliking types that are not very effective on them. For example, Cyndaquil, a Fire type, will like the Fire, Ground, Rock, and Water fields. Pokémon do not gain or lose Happiness in neutral-effectiveness fields, such as Electric for Cyndaquil. The Typeless field does not increase/decrease Happiness either. For dual-type Pokémon, both types affect the Pokémon's field preference. A Bug/Poison type will be neutral to a bug field.
Sweet Hearts, Cookies and Easter Eggs can be used to raise happiness.
- Cookies are often given out for events, Übercharm gifting or for other various reasons.
- Sweet Hearts were obtainable from the Valentines Event, it is unknown when these will be obtainable. Consumables can be used prior to 5/Aug/2014.
- Easter Eggs were obtainable from the Easter Event, and can be purchased on the Market Board now, along with Sweet Hearts and Cookies.
Trading can be used to restore a Pokémon with low happiness to 70. However, this can also lower a Pokémon's happiness if it is higher than 70.
If a Pokémon is put into a field that is not very effective against their type, they will lose Happiness. For example, Umbreon, a Dark type, will dislike the Psychic and Ghost fields. Since Umbreon completely resists the Psychic type, its Happiness will decrease faster in a Psychic field than in a Ghost field.
Use of the Red Gigaremo will reduce the happiness of both Pokémon in the Daycare while increasing the number of eggs bred. If Happiness runs low, the pair's breeding rate is decreased until their Happiness is restored.