In battle
Frisk allows the Pokémon to see the opponent's held item upon entering battle. If the foe is holding an item, a message identifying the item will be shown right after the Pokémon with Frisk enters the battle. If the foe is not holding an item, no message will appear.
In a Double Battle, an unidentified opponent is randomly chosen and their held item revealed.
Generation VI
In a Double Battle, Frisk now identifies the held items of all opposing Pokémon.
Outside of battle
Frisk has no effect outside of battle.