Yamibo - Darkness, the Hat, and the Travelers of the Books (2003)
Hatsuki is a highschool student living with her sister, Hatsumi, who she has a huge crush on. On Hatsumi's 16th birthday, she is suddenly surrounded by a green light and disappears in front of Hatsuki! She manages to follow Hatsumi with the help of a being resembling a fat baby chick (literally), en