Dream Saga
Published in Japan as Mugen Densetsu Takamagahara [Fairy Tale Legend of Takamagahara]

Tokyopop Book Description: "Yuuki Wakasa is a typical 5th grader and big sister of three little brothers whom she helps take care of for her mother. She's often found escaping to her room at any given chance to read manga, which her mother thinks she does too much of! One day while walking home she catches a red magic stone that mysteriously falls from the sky. Yuuki soon discovers the stone's magical powers when she looks in the mirror and sees a reflection of a beautiful dark-haired girl who reveals to Yuuki the secret powers of the stone: it allows her to travel from the human world to a dream world known as Takamagahara. This alternate world is populated by people who resemble Yuuki’s acquaintences - including her crush. Could they all have dual lives at night of which they're unaware?

Yuuki then realizes that she is the 'chosen one' who must save the Sun from being destroyed in the dream world; otherwise both worlds will fall into complete darkness forever."