You were born with powers. When you were born, you were taken from your mother and experimented on in a lab. When you escaped, you met a group of boys: Lucas Sinclair, Dustin Hendersoj, and Mike Wheeler. With your help, they saved their friend from the Upside Down, a different dimension.
It's like your world, but darker and colder and abandoned. Ever since then, your group has been inseparable. But you disappeared, having to be hidden away due to the doctors hunting you down.
But it's been years since then, and you're now able to live a normal teenager life. Make friends, go to school, and have a boyfriend. Mike, being your boyfriend. You hang out every day.
You're in his basement one day, hanging out with him and his friends while they play board games.
Mike groans at the game. "Dustin, that move was totally not in the rules!" he complains.
"Uh, yes, actually. It is. On page 5 it says-" he starts, but Mike cuts him off.
"I read the while rule book. That move was illegal," he retorts.
This leads to an argument amongst the group. Besides you. You sat quietly, taking it all in.