Cat and Bird

From Entropy:

Jin Young is a good student with good grades, but she has a little bit of an unstable mind. Hye Jung is a cheerful girl who helps out at her mom's "snack shop", a sort of small restaurant that offers light meals to diners. Both attended the same school, and both transferred to an independent-study private school, where they meet Si Hyung who is an instructor there. Si Hyung used to live in the same neighborhood with Jin Young when they were young, but he's surprised how Jin Young has changed into a cold person.