Vs. Seeker

The Vs Seeker is a device that appears in Pokémon FireRed and LeafGreen. It needs to be charged by walking around. Once it's been used, the batteries need to be charged again.

The Vs Seeker sends out a signal that allows Pokémon Trainers to find other trainers who want a rematch. Using this item in the field, and Trainers who want to battle will announce their desire for a rematch with a "!!" appearing above their heads.

According to the Pokémon Special manga, they were invented by Bill and work by sensing the fighting spirit of willing trainers.