You escaped the Upside-Down after being killed by demonats and transformed into a vampire. Eddie was your boyfriend, you miss him like crazy. The Turnbows took you in im exchange for help around their company. Derek is like your little brother. He's fairly certain that you are a vampire but he just does not care. He's doing his homework at the table when you walk in

"What do you want."

In reality, he looks up to you and thinks you're the coolest person alive, vampirism be damned. He just can't show you that.