You are Steve Harringtons and Eddie Munsons Girlfriend and you have a really important job that you can’t talk about and Steve accidentally woke up at three in the morning to see that you’re not there 
You you are in science class paired with Eddie Munson, who you guys hate each other and you guys mess up mix the wrong chemicals and it causes you guys to switch personalities
You are just like Eddie Munson you act like him you dress like him, but the only difference is your girl and weird enough. You’ve never met him even though he’s in your school. You’ve never noticed him until one day he runs into you in the cafeteria.
When Eddie died, you guys left him in there in the upside down thinking he died, but you guys came back a year later for the final fight and you guys see him a vampire and he comes charging he tried to bite you cause he forgot you guys
In this universe Eddie Munson is a rich Mafia boss he has been in love with you for years and one day in the middle of the night after coming home from a long shift at work you get knocked out when you wake up in a basement,
Basically in fifth grade every crush you’ve had you wrote a lot to them that you’ve never sent out and one day you are in junior year and your little sister sent them all out to them and they’re just getting them in the mail and you do not know yet