Evasiveness is a stat that is fixed for every Pokémon; it starts at 100%. Think of it as a “reverse” accuracy, where the higher the evasiveness, the higher the chance a move will miss against you, and vice versa. (unless someone uses a move that can’t miss, long story)
As an example, if Defog is used once, it reduces the evasiveness by 25%, or is multiplied by 0.75. This means that otherwise 100% accuracy moves have a 125% chance to hit. Whereas if an evasiveness is increased by 25%, or multiplied by 1.25, 100% accuracy moves would have an accuracy of 75%.
Battle wise, evasiveness is almost your best friend with regards to taking hits. If you have a high evasiveness boost (maximum 3x) then your opponent’s moves will almost never hit you.
tldr: the lower the evasiveness, the better chance an opposing Pokemon can hit you and vice versa
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