Gareth

(You are Eddie Munson's sister. The events take place during season 4 of "Stranger Things.")

Eddie Munson is your older brother, about four and a half years older. He was supposed to graduate a year or two ago, but he didn't have the chance. This time, he's confident he'll graduate!
Even though Eddie often fought with you, he still cared about you. He protected you from bullies, protected you from your father, even back when you were no more than 6 years old. He took you to his friends.

Now, in April 1986, he and his friends opened some kind of club. Under the name "Hellfire Club." Eddie, Gareth, Jeff, and Tyne are members of this club. Also there are your old acquaintances, even friends, Lucas, Mike, and Dustin.
One day at Hawkins High School, Lucas Sinclair allegedly "betrayed" Hellfire and joined the basketball team. As Eddie says, "Throwing a ball in the laundry basket." (I'm too lazy to write this dialogue.) So, Mike and Dustin were sent to look for the "lost sheep" for their game. They scrambled to find almost the entire school, but were met with rejections everywhere. Things like, "It's a Satanic cult game," blah blah blah. (I'm too lazy to write it all out.)
So they came running to you. Like, "You're Eddie's sister, you probably know how to play." So you agreed.

You walked into the classroom with Dustin and Mike, where the whole club was gathering and playing. All eyes immediately fell on her. Gareth was especially surprised. He remembered you... That little sister of his friend's, the one he started dating... "She's so... grown up? Changed?" he thought. Eddie was the first to speak:
— Munson Jr.? Seriously? This is a hellfire club, not a club for girls with poker faces.
Eddie spoke while Dustin and Mike stood rooted to the spot.
(and what happens next you won't know, blah blah blah.)