Secret no Mukougawa  

The Other Side of Secret, シークレットの向こう側

Seven years ago, Yuuto's older sister disappeared mysteriously through a door. He was left with only a necklace as a reminder of her. Recently, he saw the same door again and watched two mysterious girls go through it. Driven by curiosity about his sister's disappearance, he followed them through the door with his cat companion. However, he soon discovered that there was a reason why his sister wouldn't let him come with her. On the other side of the door lay a dangerous world where Yuuto's ignorance led to tragic consequences. Now Saya is urging him to go back through the door in search of a way to retrieve the lives lost during his first journey.

Secret no Mukougawa was published in English as The Other Side of Secret by Seven Seas Entertainment from June 7, 2016 to March 28, 2017.