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with raising the Pokemon with that Pokemon for many time You can raise your Pokemon happiness by defeating many trainers with Pokemon do you want to raise.
Maxing out the happiness of a pokemon is somewhat time consuming. Walking around with the pokemon in the lead position will help slowly increase the happiness. You can also feed the pokemon poffins to help raise stats and happiness.
you have to get a poke app . it shows your pokemons happiness in hearts
you can check a pokemons happiness in Diamond/pearl by the use of a poketch app, in previous games you could only know in the happiness was maxed by obtain a ribbon from a certain character depending on the game.
You have to walk around with it A LOT!!! But if you catch it with a luxury ball it helps a lot.
in one of the citys in a house a woman says shell talk to a Pokemon when shes done you pokemons happyness has rose
right next to the pokemons name
behind your pokemons
in goldenrod city are a very small house right and down of the city in this house are a little girl a old man a in the middle of the house are a lass what tell the happiness of your first Pokemon in your party
to find happiness -- trouver le bonheur
You can find legendary pokemons after you battle the gym leaders and the elite 4.
Letting it out of the pokeball to go on walks with you. Giving the pokemon Soothe Bell. Battling other pokemon and winning. Feeding it Rare Candies.