If the Defending Pokémon doesn't have a Char counter on it, flip a coin. If heads, put a Char counter on it. A Char counter requires your opponent to flip a coin after every turn. If tails, put 2 damage counters on the Pokémon with that Char counter. (Char counters stay on the Pokémon as long as it's in play.)
If the Defending Pokémon tries to attack during your opponent's next turn, your opponent flips a coin. If tails, that attack does nothing.