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I really don't think Akutagawa forgot who Atsushi was. When he said "Who the hell are you?" I think he was referring to how moments ago Atsushi yelled at him that it's no use. Akutagawas number one source of respect for Atsushi comes from that unyielding determination of his, to Akutagawa, it was his number one defining quality. It was what made Atsushi, Atsushi. Now he's seeing that same person yell at how it's no use while Akutagawa now has the capability to fight back, and Atsushi was groveling on the ground in front of Dostoyevsky moments ago. He doesn't feel like the Atsushi we all grew to know and love throughout this whole series. Akutagawa asking that question jot literally, but in the rhetorical "Where has the quality that made you, gone? Who are you without that blinding determination that fights even in futility?" Besides, when I see one of my friends go through a drastic change, I'll jokingly ask them "Well who are you?" to tease them that they changed a lot. So when I read that, I automatically went, "Oh, Akutagawa is seeing how much Atsushi has changed since he left, and that's just how he expressed it." I don't know, with Akutagawas personality, I don't think any effects of vampirism can erase the people around him, at least not entirely. Maybe it made him forget some of their nuances, but the person entirely? Well, I don't think he'd really be Akutagawa anymore. Besides, you can't tell me he'd forget Atsushi when their abilities fought on sight and when Akutagawa died for him. That's a bond no amount of weird ability shenanigans can destroy.