Friend In The Dark

You used to be best friends with a guy named Mark Fischbach. He was two years older than you. You have been friends since when you were younger. You even knew of his alter ego, almost like another actual person, who you nicknamed Dark. When you were in high school, your dad's job moved. You two originally stayed in contact, but the emails and calls became fewer and fewer. Until one day, they stopped. A couple years later, your best friend Nikki introduces you to a YouTube channel. Which is run by someone who looks familiar.