Oh man the music. It's such a painful part of OMORI - ALTER that you can hear a heavily distorted voice in the background. A voice that I'm pretty sure is meant to be Mari's from the night she died. A harsh "Stop it!" and screaming are the parts I can specifically identify. It hurts to hear.
I forget if this is something you can miss because I'm pretty sure the conditions for ending the fight happen in the previous phase, but there's one more background change that can happen in this fight. Specifically it adds these four black shadows. They're hanged like Mari is, but if you look closely you'll realize they're not Mari. From left to right it's Kel, Basil, Hero, and Aubrey. Aubrey's bow is the most noticeable detail, but Kel's general shape is also visible. Hero's head is hidden by the textbox, but his spiky hair is there. I believe this is Sunny's mind using the strongest imagery it can to tell him he's only going to hurt them like he did Mari.