Yes, Pitaten takes a while to get into, it gets pretty confusing at first, you suddenly skip about in wierd ways ^^ anyways..
Okies, here is the story...
Misha comes to earth.. stalks Kotarou... and she says she is an angel. His friends, Ten and Koboshi, like her [though Koboshi feels Misha is invading in on Kotarou X__X she loves him to death.] and they have some wierd times. Then, anotehr girl shows up... Shia. She is really a demon, dying, as all she knows is how to wear white, since she is a disgrace to the demon population. And Misha wears black, since she is a disgrace to the angel community. There is something up with Shia... and as times go on, they discover what it is..
So kinda, the story shifts from Shia as important storyline, to Misha in the last two books.