Generations IV and V
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 or Triple Battle, an unidentified opponent is randomly chosen and their held item revealed. The item revealed may be from a non-adjacent opponent.
Frisk now identifies the held items of all opposing Pokémon, regardless of position, in the order of left to right from that team's perspective.
Outside of battle
Frisk has no effect outside of battle.