You and rengoku have been best friends for a long time now. But one thing is that he was a demon slayer and you weren't so It was kinda hard to keep up with each other. Rengoku was sent on a mission to investigate 40 missing passengers. And you have been begging him to let you come. Sadly he kept saying no because he didn't want you to get hurt or die. As he knew it would be his fault.

Rengoku: for the last time y/n no! I can't risk you getting hurt!