Flat is a manga series by Natsu Aogiri. It was serialized in Comic Avarus.
Heisuke is a young man who was left in charge of taking care of his little cousin Aki for a day. Being an only child who usually stays at home alone, Aki is quiet, reserved, and will go out of his way not to make trouble for others. As they spend more time together, Aki begins to grow very attached to Heisuke, especially since Heisuke's hobby is to make sweets and desserts. The quiet Aki eventually opens up to Heisuke, and Heisuke in return teaches Aki that sometimes it's all right to be selfish and say what you really want to do.