Mantis is an human/plant hybrid who was raised in Vietnam. When she met the Avengers, she fell in love with the Swordsman (Marvel), and she joined the team. She left Earth when Swordsman sacrificed himself to save the Avengers, but temporarily returned to marry his ghost. She then joined the Guardians of the Galaxy. Mantis uses her powers to protect the galaxy against those who would seek to harm it.
Marvel Cinematic Universe
Mantis is an empath with the ability to sense other people's feelings and alter them. She was raised by Ego and later recruited as a member of the Guardians of the Galaxy. Since she was a child, Mantis helped ease Ego's neurotic behavior with her powers until the Guardians came to his planet. After bonding with Drax the Destroyer, she came to care for the Guardians and then allied with them to help prevent Ego from conquering the Galaxy. With her own servitude to Ego ended, she became a full-fledged member of the Guardians and accompanied them on further adventures.
When the Guardians were called to action to stop Thanos from collecting the Infinity Stones to wipe out half the universe, Mantis accompanied some of the Guardians to Knowhere where they failed to stop the warlord from gaining the Aether resulting in Gamora being captured. After, they traveled to Titan where they teamed up with Doctor Strange, Iron Man and Spider-Man to fight Thanos head-on. The fight once again ended in failure, and Mantis and her fellow Guardians, except for Nebula and Rocket Raccoon, were killed when Thanos obtained the Infinity Stones and snapped his fingers.
Fortunately, five years after the Snap, the remaining Avengers successfully gathered the Infinity Stones from different points in the past, and Bruce Banner undid the effects of Thanos' snap, bringing Mantis back to life. She then participated and survived the Battle of Earth, attended Tony Stark's funeral after he sacrificed himself. Mantis then resumed being a part of the Guardians, with all of them reunited again, now joined by Thor.