Missing Blue (2001)

At Seiryou high school, second year student Masamura Kouji leads a stready, but busy life, alongside with humble and quiet childhood friend Ritsuki Saya and senior student Nishizuku Ruuna.

One day, a transfer student, Kasuga Mizuki, appears and hands over a mysterious crystal while uttering enigmatic words : "Please remember...".

While being certain the two of them never met, the main protagonist feels they somehow did. A peculiar connection?