Costar

Costar copies all of an ally's current stat changes when the Pokemon with this ability enters the field in a double battle. If the ally has boosted its Attack and Speed by two stages, the Costar Pokemon immediately receives those same boosts upon switching in.

The ability copies both positive and negative stat changes from the ally. It activates only once — at the moment of entry. Any further stat changes to the ally after Costar has activated are not retroactively copied. In singles battles, Costar has no effect since there is no ally to copy from.

Costar is the signature ability of Flamigo, the Flying/Fighting-type Pokemon introduced in Generation IX. It enables strategies where one Pokemon sets up stat boosts and then Flamigo switches in to instantly duplicate them, effectively doubling the team's offensive momentum without spending additional setup turns.