Edward himself could be the main character. Maybe he's on a hunt or something and he spots a human or another vampire and he has that sense of 'I know those eyes'. His vampire abilities might give him some sort of premonition or recognition when he sees certain eyes. It could also be related to his past or some sort of vampire lore that he's aware of through the eyes he sees.
Alice could be the main character in this fanfiction. Given her ability to see the future, she might see visions related to a pair of eyes. And when she actually sees those eyes in person, she says 'I know those eyes'. She could then use her knowledge to either prevent something bad from happening or to bring about a positive change in the Twilight world.
It could be about a character in Twilight who recognizes someone by their eyes. Maybe it's a love story where the protagonist spots their soulmate just by looking into their eyes. It might also involve a mystery, like a character having special powers related to eyesight and they use it to uncover secrets in the Twilight universe.
Jacob could also be the main character. After all the chaos with the Cullens and his own werewolf pack, he might long for the days when he was just a normal boy in La Push, before he knew about vampires and before his life got so complicated with his love for Bella and his duties as a werewolf.
Maybe it's a long - lost relative of Bella Swan. She has some knowledge about the vampire - werewolf world that Bella didn't have, and she's under the bed because she's trying to avoid being detected by some unfriendly vampires who are after her for some reason.
It could be an outsider who manipulates Twilight's friends. This character might be envious of Twilight's popularity or power in the magical realm. They trick Twilight's friends into believing false things about her, making the friends turn against her.
It could be a new vampire created by Carlisle who finds his perfect match among the humans in Forks. Maybe a shy girl who is intrigued by his old - world charm.
It could be someone completely new. A character that the author creates to disrupt the status quo in the Twilight universe. This new character might have unique powers or abilities that change the course of the story.
Since it's a fanfiction about a 'perfect stranger', it's likely that the main character is this stranger. But without reading it, it's hard to say more about who they are exactly.
It could be a new character, like a young girl who has just moved to Forks and discovers she has psychic powers. She then gets caught up in the existing drama of the twilight world.
It could be that the main characters are the Cullens as a whole. In many Twilight - related stories, the Cullen family often takes center stage. They might be the ones dealing with the 'devil' situation in this fanfiction. Or perhaps Jacob and his werewolf pack could also be main characters if the story focuses on the werewolf side of the Twilight universe and their interaction with the 'devil' - like elements.
It could be a new character. Maybe an orphan who discovers they have these reanimation eyes and has to figure out how to use them while also dealing with the fact that they are different from everyone else in the ninja world.