Costar
Copies ally's stat changes on entering battle.
Full effect
When the Pokémon enters a battle, it copies an ally’s stat changes.
In game description
When the Pokémon enters a battle, it copies an ally's stat changes.
Pokemon with Costar
Using Costar in Cobblemon
Abilities are passive and always active, so Costar costs you nothing to run. If it is marked hidden on a species above, expect it to be harder to obtain: servers differ in whether hidden abilities come from breeding, an Ability Patch, or events.
See the Pokedex for which abilities each species can have, and IVs, EVs and breeding for how to actually pass one down.