Atsushi briefly mentions in episode 4 as a child of having an older sister, who he fondly referred to as "Nee chan" in the anime. Not much is known about her, except for the words told to him as a child, "a wish made on a shooting star thrice before it disappeared will come true".
Kinshiro KusatsuKinshiro were best friends during their childhood. As revealed in episode 11 of the anime, it is discovered that the reason for their distance is that Atsushi went for curry with En instead of going to his house after school. This lead Kusatsu to believe Atsushi had rejected his friendship, causing him to become distant. However Atsushi was oblivious to what he had done and thought it was best to not force anything out of Kusatsu, therefore he started spending more time with En, and shortly after must have become best friends judging by their current relationship. Atsushi is also polite when speaking to Kusatsu, showing that he still respects him and the friendship they once had.
While they were still friends it is shown that the two made wishes on shooting stars. Kinshiro's was that they would stay best friends forever. Atsushi assured him they wouldn't need a shooting star for that as they would always be friends.