Ryusei Umiushi

Mimi, a high school girl, has a childhood friend, Ren-kun, who is a working adult. Ren-kun is a bad adult. He pretends to go to the office and continues riding the subway to the last stop, or calls Mimi up in the middle of the night and asks her to make a cake. A relaxed life with an older childhood friend that will loosen your heart when you read it. (Source: Hakusensha, translated)