Happiness is something of an invisible statistic for Pokemon. It is used to gauge the power of the moves Return and Frustration. Return gains power the higher the happiness level of a Pokemon is. Frustration is the exact opposite of Return. Happiness levels also allow certain Pokemon to evolve once a certain level of happiness is reached. There are numerous ways to aid in getting a Pokemon more friendly(AKA happier) Feeding it the EV Berries(Pomeg, Kelpsy, Grepa, Hondew, Tomato are some), giving it vitamins such as Protein and Iron, keeping it in good health during battle, letting it walk with you(HG/SS only), and keeping it in the first team slot in your party are all ways to do this. There are also 2 kinds of Pokeballs that help happiness grow quicker. The Luxury Ball makes a Pokemon grow more friendly quicker than most other Pokeballs would do. A Friend Ball(HG/SS only) will make a wild Pokemon be happier from the start, but will not effect the rate at which happiness grows after that.
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