Mirai and Ahikito's fight. The one person to treat her like a human being runs the risk of permanently becoming an immortal rage monster and she has to kill him before this happens. The scene is Deliberately Monochrome and she has flashbacks to her most cherished moments with him before finally delivering the fatal blow.
In Episode 11, there's the scene where Akihito reads Mirai's last text message to him. She explains her decision to sacrifice herself to Beyond the Boundary in order to let him live, saying that even though she knows he'll be upset with her for this, she's already made up her mind. She thanks Akihito for being her friend and all the times they shared together, then tells him that he can live a happy, normal life from now on. The text message ends with the sentence, "I don't feel unpleasant anymore." Akihito breaks down crying.
Episode 0 has a brief shot of an extensive first aid kit and several strips of bloody bandages in the derelict house Akihito resides in, alluding to just how many people have hurt or tried to kill him in ignorance of his human emotions.
Episode 0 as a whole for Akihito. His entire outlook is a jarring contrast to the relatively upbeat person seen throughout the main series. He has no friends, no hope for his future, he's treated as an abomination and it's plainly obvious many Spirit Warriors have found him before and have tried making attempts on him. He doesn't bother fighting back against Hiro'omi because he doesn't want to hurt anyone and he wishes Hiro'omi would kill him just to put him out of his misery. What's worse though is that no matter how badly he wants to end his life, he literally can't.
In the Mirai Hen movie we learn a little bit more about the last days of the Cursed Blood clan. It was a genocide. Mirai's mother and another woman who is presumably Mirai's grandmother or aunt were just running away from Spirit World Warriors who called them monsters, told them to die, and murdered them. This is all despite them never fighting back.
Also from the movie, there's Akihito's depression over Mirai losing her memories. He constantly tries to avoid her and has a pained expression whenever he sees her, unable to accept that she's forgotten about him and all they went through together. When Mirai confronts him about this, he insists that he just happened to pass by when she regained consciousness on the roof, and that the reason he broke down in tears upon seeing her was because of his glasses fetish. Despite all of his friends telling Akihito to tell Mirai the truth, he stubbornly refuses because he feels he'd be robbing her of living a normal life. And when Mirai starts to regain her memories after visiting a park she and Akihito had gone to in the past, Akihito tells her that she is "the most important person in his life", but that they can't be together because she'll just be unhappy with him.
