In a recent interview, he explained that Mantis, played by Pom Klementieff, wasn’t exactly as he and co-writer Don Heck wrote her.
“Well, I was not happy with Mantis’ portrayal,” Englehart said. “That character has nothing to do with Mantis.”
So what’s the problem? For the people who haven’t read the comics, apparently movie-Mantis is similar only in look. In the comics, she’s a human raised by a Kree cult. Once she was trained, her memory was wiped. In the film, however, she’s essentially Ego’s caretaker.