Need help finding manga...

A while back, I was reading a manga. Never got to finish it and I really want to, now. Problem is I don't remember what it was called. I don't have much information about it, but hopefully someone can tell me its title. The manga was about a girl who traveled demon/devil. He only transform into a demon when he needs to. At one point, the girl's sister finds them. She seems to be in love with the sister that is traveling with the demon. She gets all kind of weird, perverted thoughts about them. That's pretty much all I remember. I hope someone can help me find this manga.


--Found it. Google saved my life. "Crimson Grave"