Glen got his nickname from his birthmarks that painted his face.
Glen was stationed in the EU before the outbreak occurred. He had been trying to make more money and was seeking more opportunity. He didn't anticipate the shitstorm that was Chernarus when he got restationed after the outbreak occurred to assist in research and guarding scientists for the company he had been working for the past few months.
He was assigned this position because of his past experience in the military as a United States Ranger of the 75th Battalion. He wasn't certain of the goals of this organization, but it wasn't like him to ask questions. He kept his nose to the grindstone and did what he was told.