While she wasn’t around for the first Guardians of the Galaxy movie, Pom Klementieff’s Mantis was quickly established as an important character in Guardians of the Galaxy Vol. 2, with Ego the Living Planet, Peter Quill’s father, relying on her empathic abilities to help him sleep while he was in his human form. After Ego was defeated, Mantis joined the Guardians and was present for the events of Avengers: Infinity War, but was dusted during The Decimation. Fortunately, she and everyone else snapped away were brought back to life in Avengers: Endgame.
Article source: https://www.cinemablend.com/news/2485157/the-next-two-mission-impossible-movies-have-added-a-guardians-of-the-galaxy-star