How is Nia a Mary Sue? She is a MAIN CHARACTER; the MAIN HEROINE at that. She obviously has presence and importance to the narrative. Please, stop just blatantly trolling while saying "SHE IS A MARY SUEEEE, BAAAAW" and please try to make a point.

Also, you're blind if you thought she did nothing to help Simon. She understood his feelings and gave him a PURPOSE. She taught him it's completely fine to run away, that it's fine to feel fear and that HE IS JUST A HUMAN.