Marshadow (マーシャドー)

Marshadow is a major character in the movie, Pokemon: Kimi ni Kimeta!. Marshadow is said to be a guide from the shadows who leads those chosen by Ho-Oh to be the Rainbow Hero. If the Rainbow Wing given to the chosen person loses its color, Marshadow is tasked with sealing the Rainbow Wing away.

(Source: Bulbapedia)