With her emphatic abilities and quirky personality, Mantis proved to be a solid addition to the Guardians of the Galaxy Vol. 2, providing major laughs and emotions. Mantis was essentially raised by Ego to be his own personal ZzzQuil, but once you learn Ego's true motives, her origin comes into question. One fan theory believes that there's more going on with Mantis' backstory than the film is letting on, and it all ties back to Star-Lord.
Obviously, SPOILERS, so bookmark this page and come back to it if you haven't already seen Guardians of the Galaxy Vol. 2.
One Reddit user believes that Star-Lord and Mantis are actually brother and sister, with Ego as their biological father. This is never explicitly said in the movie, nor does anyone point out the sort of familiar relationship they already have thanks to Ego raising Mantis. However, the theory makes quite a case for itself.
In the Marvel sequel, Mantis (Pom Klementieff) is an empath whose unique abilities allow Ego (Kurt Russell) to rest and recharge his human avatar. As Mantis tells it, Ego found her in her larvae stage and took her under his wing. Their relationship is more master-servant than father-daughter, with Mantis serving as an unwilling accessory to Ego's true plan: remaking the galaxy in his own image. Part of the way he intends to do this is through the aid of a child, so every time he visits a planet, he impregnates at least one woman, which eventually leads to the creation of Peter Quill/Star-Lord (Chris Pratt).
So, knowing all that, it doesn't really make sense that this literal egomaniac would rescue Mantis, even if she does serve a vague purpose.
The theory goes further on to speculate that Mantis is another one of Ego's children from his many...uh, explorations -- only this time he took the child with him because of her eventual abilities to allow him to sleep. The only reason that she doesn't end up as another skeleton in his basement is that she serves a useful function to him. He simply never told Mantis this, probably because even Ego is self-aware enough to know that she probably wouldn't be down to help him nap if she was just another one of his children that always get murdered.
It's a solid theory, and it makes sense from both a plot and theme perspective. As the theory creator points out, lying to his children is kind of Ego's thing, and while his self-serving attitude would totally keep Mantis just for her powers, her presence there is still pretty vague.
Hopefully, James Gunn gets wind of this theory and sets things straight; he's usually pretty good at that sort of thing. In the meantime, Guardians of Galaxy Vol. 2 is in theaters right now.