She is one of five Trainers who can join with the main character during an event in Pokémon Diamond, Pearl, and Platinum. Later, she can join the player in competition at Sinnoh's Battle Tower. She first appears in Eterna Forest. She asks the player to join her because she's afraid she might run into Team Galactic. Her Chansey is good at using restoration techniques like Refresh and Softboiled, but isn't good at offense.
In Diamond and Pearl Cheryl is the only double battle partner the player must team up with to advance through the game. She is also the set default. This means that if ever a player uses a walk-through-walls cheat to go through the places where the other stat Trainers should be for the first time, Cheryl will be there as the partner.
However, this was changed in Platinum, so Riley must be encountered to collect a HM on Iron Island.