When Nezumi Informant enters the battlefield, each opponent discards a card.
To resolve Nezumi Informant’s ability in a multiplayer game, the next opponent in turn order (or, if it’s an opponent’s turn, the opponent whose turn it is) chooses a card in hand and sets it aside without revealing it. Then each other opponent in turn order does the same. Finally, all chosen cards are revealed and discarded at the same time.