Atsushi and Kinshiro are childhood friends, as seen in a flashback, however this friendship does not seem to be present at the moment.
Kusatsu and Atsushi as children.
After Kinshiro and Atsushi's friendship started to grow weaker, he and En got closer. It is shown that their friendship developed after they had curry together because the both had 20% off coupons. Both enjoyed it and to this day, their friendship remains.