God, I love this author. She takes an Amnesia trope, normally used to drag out a series by revisitng long-resolved conflicts, and actually turns it into a progression catalyst. Both Utsumi and Nachi had given up on each other, but with Utsumi's return, it sparked the hope to return to Nachi, but Utsumi still wanted to run away from it all. Thankfully with Nachi reverting to a childlike mindset, it sparked in him a desire to take care of her (probably partially cause he felt he was at fault for the accident), and in doing so was able to relive the moment when someone accepted him, tail and all, which he'd been needing ever since he left Nachi. I'm sure he thought her feelings might have changed in all those years since he'd left and wanted to keep his happy memories of when someone did accept him and not take the chance to ruin it by founding out Nachi changed. Thankfully he stuck around just long enough.
It's just SO GOOD