http://archiveofourown.org/works/3727768
Summary:
Everyone's born with their soulmate's first words to them written on their skin, and that should make things easy, but it really doesn't.
"Roy has alternatively despised his mark and clung to it like a lifeline. Colonel. Just one word, right at the back of his foot. His literal Achilles' heel."
Review:
Edward knows that Roy is his soul mate, but he couldn't let Mustang know, he had to fix his mistakes first. He had to keep his promise to Alphonse. So, Ed uses the most generic word/phrase he can think of, so Mustang won't know. Ed keeps this a secret for years and finally tells Roy after all that.