Munna has willingly sworn loyalty to Kyurem, assisting it in its quest to protect the Bittercold, ensuring the destruction of the Pokémon world, which Munna considers to be a world where only the honest suffer. Together with her friends, Munna successfully manages to defeat every (transformed) human being that was called to the Pokémon world by Hydreigon (the Voice of Life), sending them back to the human world, except for the player.
After the player enters the Pokémon world, transformed, Munna and her friends keep Hydreigon from meeting them, and through repeated dream visions orchestrated by Munna (starting with Munna taking control of Hydreigon's voice during his initial plea, accompanied by a fake vision of Munna being chased by Hydreigon), the player is led to believe that they were called to the Pokémon world in order to save Munna from Hydreigon. When the player and their partner find Munna, in a location far away from Pokémon Paradise, she abruptly cuts the act and has the two surrounded by her team. In their escape from this encounter, the player is joined by Hydreigon, but is separated from their partner, who is subsequently taken hostage by Munna's team. Seeing through a trap set up by Munna, revolving around the use of Frism voice recording, the player and Hydreigon are able to rescue the partner and escape.
Munna's final encounter with the player's team occurs inside the Glacier Palace, where she partakes in the final showdown between her team and the player's team. After being defeated, Munna engages in dialogue with the player's team about the fate of the Pokémon world, explaining how she and her friends were thrown into despair after being forsaken by the world. She eventually starts to waver in her resolve, but nevertheless gets ready to fight the player's team once again. However, they are interrupted by Keldeo, who appears alongside Umbreon and Espeon via a Magnagate, having been freed by the two. It is revealed that Keldeo had been a prisoner of Kyurem and Munna's team, in hope of convincing them to change their plans.
As the dialogue continues, with Munna further persuaded by the player and partner's conviction, tremors cause a huge icicle to collapse toward the player and partner. Munna jumps in and saves them from the terrain-splitting icicle, barely escaping it herself. She encourages the player and partner to confront Kyurem and save the world, declaring that she will not participate, as she has sworn loyalty to Kyurem. When the player is fighting the Bittercold, Munna is one of the Pokémon sending her "Don't give up!" thoughts to the player, weakening the Bittercold. Like the other Pokémon who were at the Glacier Palace as it crumbled after the Bittercold was destroyed, Munna is implied to have been saved by Kyurem.
She later reappears in Super Mystery Dungeon, and provides the player with a letter that has to be delivered to a Gigalith. She becomes a playable character once the message is successfully delivered. Speaking to her has her express remorse for previous actions, and that she wishes to make up for her mistakes.
Dedicated to her cause, Munna is portrayed as a calm and collected Pokémon, displaying no signature phrase/exclamation/onomatopoeia in her dialogue, aside from the occasional, recurring Heh-heh-heh... (Japanese: フフフ・・・・・・ Fufufu...), unlike many other characters in the Pokémon Mystery Dungeon series, including several of her teammates. She is well-respected as the leader among her friends, who refer to her as Lady Munna (Japanese: ムンナ様 Munna-sama). Although initially depicted as a cunning, deceiving leader accompanied by her gang of thugs, Munna and her teammates are shown to care deeply for each other as the story progresses. She is capable of communicating telepathically with Kyurem, and performing various other acts of out-of-battle mind influence.
In Super Mystery Dungeon, she is apologetic for her previous actions, and wishes to atone for her previous sins.