From Whisper:
Yonekura Mugiko, senior in high school, is a talented exorcist dedicated to her work. One day, she was accidentally pushed and fell down a flight of stairs, resulting in amnesia! She only remembers things up till spring of her 6th grade year, hence: she has the mentality of a grade school kid… and forgets about her boyfriend, Kakurazaka Hikaru, a well-known exorcist as well. Mugiko and Kakurazaka will work together to exorcise spirits whenever they are hired, despite the little accident. Meanwhile though, Kakurazaka, known for being a perverted hothead, will try to regain Mugiko’s memory back… to be romantic with her once more!!

A princess who holds a piece of a legendary treasure map known as the "Stone of the Sea" willingly joins a famous pirate in order to avoid becoming the twelveth wife of some 72 year old king...

(~from manga-mania)