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From Shoujo Manga Maniac:

Kojima Mitsu (a.k.a. Honey) is in middle school (She's 12 years old) but she's so small that she looks like she's in elementary school! One day while she is is changing into a maid uniform for a school event (with not much success…the uniform is too big for her), she find a little bat blocked by the window and helps it. Suddenly an awesome guy appears in front of her! He is a vampire and as thanks for saving his "toy," he helps turn her into an adult (with a kiss)…