Tenshi no Su

From Tokyopop:

A woman named Natsu finds Ken, her husband, with another woman when she comes home from work one day. Ken confesses that he is in love with the girl, mi, and asks Natsu for a divorce. Natsu doesn't really feel shocked OR sad, for some strange reason. All she can think of is what she has learned from her five years with Ken, and how she can move forward. Then, one sleepless night, she sees an angel sitting on her balcony…and soon after, the angel begins living with her.