Steven here but always go by Steve.
I had a friend in Jr. High/High School whose name was Steve, too.
When he was in 7th grade, he had the start of a real scraggly dark beard/mustache. Think like Shaggy from Scooby doo, except patchier and really dark. Somebody started calling him Fungus. He was a great guy, very popular, and he laughed and embraced it. It stuck.
For the next few years, that's all everybody called him. It got to where lots of kids knew who Fungus was, but had no idea that his real name was Steve. His mom would go ballistic when somebody would call his house asking if Fungus was there.
Finally, we just started calling him Gus for short by about his Jr. year, and that's been what everybody calls him since.