From Turtle Paradise:

It's not that she tries to keep to herself, exactly: It's just that everyone seems to look right through her! Naturally, Sakuya Tohno has a complex about it. She says she's just like air: ever present, but never seen. All she wants is a friend, but building relationships isn't exactly easy for someone who is, for all intents and purposes, invisible. But then she hears a legend about the "Thousand-Year Sakura" and how, if one makes a wish beneath its perpetually-blossoming boughs, it's sure to come true. So in a moment of desperation, she races to the tree, only to find four very attractive boys waiting for her... And at that moment, beneath the cherry blossoms of the ever-blooming Thousand-Year Sakura, something incredible happens!