Crimson Prince, Volume 9 (Crimson Prince #41-45)

Once upon a time, a vocal young Mutsuki was insistent that demons are far from evil, to the scorn of her fellow angels. What's more, she was ready and willing to prove it - by venturing into the demon realm to meet one in person! Mitsuki will never forget how her sister suffered as a result, but these days, she's finding herself questioning her belief that an angel could never be friends with a demon...