It's obvious to me Kanna had something to do with the weird dynamic between Koharu and Aki. Considering she talks about stockpiling ammo to fast track Koharu to human again, I almost wonder if she hit Aki intending to get both of them and ran out of ammo and something prevented her from finishing the job. She's also present and "disguised" in the beginning... Actually, the rule of protagonists often means Koharu is special. I wonder if Kanna actually shot her and it didn't work, and it became a chip on her shoulder all this time creating that one sided situation. That this time for sure she's going to make sure she falls in love with someone.
Or rather, it seems like Kanna was attaching herself to Koharu, using her couple setups as a way for quick karma? Maybe she was really single-minded in getting back to being a human. Koharu would set up the pairing and Kanna was right there to make sure it happened. She could have been using her, delaying her attraction to Aki to make the most out of what ended up being a useful pawn, a human wingman so to speak. And once that whole accident unfolded, it probably shook her pretty hard.
