>>100945754I mean, if she is not Ego's child, then it leaves a gap in the story.
Why would Ego pick up a random bug child like Mantis if she wasn't already his child?
It's most likely that he picked her up, and though she didn't have his power, she was able to put him to sleep, which he found useful and kept her around. Her being his child is sort of the catalyst of her being on Ego.
