Secret
"Lily, please believe me! This is something I cannot tell you of my own choosing!"
Lily had never seen James so flustered, so inexplicably resolute, as he was now.
"James Potter, why can't you trust me?"
"Because it's not my secret to tell, Lils!" He all but slammed his head into his hands, getting up to pace around the empty common room. He suddenly stopped by the window, looking up at the nearly full moon.
"Lily Evans, I love you. You must know that by now. And you should also know that I keep nothing, absolutely nothing from you about myself. There's only one thing I've kept from you since we started dating." He paused, breathing hard and running his hands through his unruly hair nervously. "And that secret is not mine to tell."
Lily watched as her boyfriend flopped onto the overstuffed couch, clearly conflicted about their current heated discussion. She carefully sat down next to him.
Taking his hand in hers, she asked him quietly, "James? Have you ever heard of a werewolf named Moony?"
James' head shot up.
Lily smiled knowingly, and nodded her head. "Remus has... Prongs."
"How did you..."
"I figured it out. And then Remus confirmed it. Now," she shifted closer to him. "Tell me more about Moony, Wormtail, Padfoot, and Prongs."
I always figured James wouldn't tell Lily about their full moon escapades unless Remus spilled the beans first. He apparently considered it the height of dishonor to betray your friends. So I don't think he'd even let it slip to Lily.