I have a fainted Abra with jolly nature and Synchronize at the front of my party. When I caught Necrozma, it had a naive nature. Does Synchronize only work on gift Pokemon? I'm doing this so I can get a physical Ultra Necrozma.
From Pokémon Emerald onward, if a Pokémon with Synchronize is in the first place in the party (even if fainted), any wild Pokémon encountered has a 50% chance of having the same Nature as the Pokémon with this Ability. Starting in Pokémon Omega Ruby and Alpha Sapphire, if a Pokémon with Synchronize is leading the party, gift Pokémon (except Pokémon obtained via Mystery Gift) in the Undiscovered Egg Group will always have the same Nature.
Necrozma is in the Undiscovered Egg Group, but it is not a gift Pokemon.
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