The final girl is a trope in horror films (particularly slasher films). It refers to the last woman alive to confront the killer, ostensibly the one left to tell the story. The character in question tends to follow a certain set of characteristics.
Is it canon that Lena actually hates aliens? I've seen that tossed around in a couple of recaps and I never got the vibe that she hated aliens at all. Apprehensive of them and their potential powers, sure. But hating them seems strongly like other the other Luthors that she's trying to distance herself from. That and she doesn't want to be judged so how could she justify judging the entire alien population, excluding Supergirl?