Urushihara, a.k.a demon general Lucifer, is a character of underlying mystery. He never talks of his past as an angel or of his times under Maou's reign. To put it simply, no one really knows much about him, when you actually think about it. This is what I think his past could've been.

To find him waking up in cold sweat after a nightmare was no rare occurrence for them, but no matter how much he pried into it, he wouldn't tell him about it. Jean gave a soft smile.