Marshadow is a Mythical Pokémon classified as the Gloomdweller Pokémon, possessing a unique dual Fighting/Ghost typing. Its physical form consists of dark, wispy smoke-like material with two glowing ember-like eyes. Demonstrating a cautious and reclusive nature, Marshadow conceals itself within shadows to avoid direct interaction. It can merge with or inhabit the shadows of other beings, perfectly mimicking their movements and abilities while potentially executing their techniques with enhanced power.

In *Pokémon the Movie: I Choose You!*, Marshadow serves Ho-Oh as a "guide from the shadows," tasked with observing and testing individuals holding Ho-Oh’s Rainbow Wing feather to determine their worthiness as the "Rainbow Hero." It stalks Ash Ketchum covertly after he receives the feather, hiding within his shadow throughout his journey. Marshadow monitors Ash’s actions and emotional state, focusing on the purity of his heart reflected in the Rainbow Wing’s condition. When Ash experiences doubt and resentment after a defeat—wishing he had chosen a different starter Pokémon—Marshadow whispers to him, inducing a nightmare where Ash inhabits a world devoid of Pokémon. This corruption turns the Rainbow Wing black, though Ash’s reaffirmed bond with Pikachu restores the feather’s color.

During the climax, after rival trainer Cross steals and corrupts the Rainbow Wing with malicious intent, Marshadow seizes the feather and unleashes its power to mentally control nearby wild Pokémon, directing them to attack Ash and his companions. Ash sacrifices himself to protect Pikachu from the onslaught, resulting in his temporary disintegration. Marshadow holds the disintegrating feather until Pikachu’s grief-powered lightning disrupts its control, freeing the corrupted Pokémon and indirectly facilitating Ash’s revival through the restored Rainbow Wing. Marshadow then silently observes as Ash completes his encounter with Ho-Oh, fulfilling its duty as a neutral arbiter of the Rainbow Hero prophecy.

Marshadow’s capabilities include the signature technique Spectral Thief, which steals an opponent’s stat enhancements, and its exclusive Z-Move, Soul-Stealing Seven-Star Strike. During intense actions or when executing this Z-Move, Marshadow temporarily shifts into a "Zenith Form," igniting its head in yellow-green flames—a visual transformation symbolizing a moment of unveiled power or purpose.

Outside the movie, Marshadow appears in *Super Smash Bros. Ultimate* as a usable Pokémon within the "World of Light" mode, assisting fighters who summon it.