As a Pokémon expert with extensive knowledge of the game mechanics, I can provide you with several methods to increase a Pokémon's happiness in Pokémon Diamond.

Happiness in Pokémon games is a measure of how attached a Pokémon is to its trainer. There are various ways to raise it:


1. Walking with your Pokémon: If the Pokémon is in your party, it gains happiness over time.

2. Using items: Certain items like Sweets (found throughout the game or given by NPCs) can increase happiness.

3. Vitamins: Items like HP Up, Protein, Iron, etc., can also increase happiness when used on a Pokémon.

4. Affection actions: In Diamond, you can use the Affection actions (like Petting or Praising) at the Pokémon Day Care to increase happiness.

5. Leveling up: Pokémon gain some happiness when they level up, especially if they are in your party during the battle.

6. Winning battles: Winning battles with your Pokémon in your party can also increase happiness.
7.
Moomoo Milk: While it usually decreases happiness, it's worth mentioning for completeness.
8.
Happy Trainer: In some Pokémon games, there's a Happy Trainer who can increase happiness, but this feature is not present in Diamond.

Remember that using certain items or actions, like using a Pokémon that isn't in your party to win a battle, can decrease happiness.

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4 AnswersEquip it with Soothe Bell. ... Give it Vitamins and Berries*Just having the pokemon on your team can also increase it's happiness so make sure and do that.If you are playing one of the 4th Gen games you can view it's happiness by using one of the Poketech apps.Give the pokemon massages.read more >>