Based on the fact that Benimaru said that Amaterasu "felt bloody" : fic idea!
Basically, what if teenage Benimaru, feeling this bloody thing, wondered what it was about and went to check?
Landing on the Amaterasu itself, he feels something else coming from inside. And it's rage. He's never felt such rage. And it feels kind of sentient. That's not very good.
So Benimaru begins to talk to it. He figures, being angry like that all the time must be very tiring. And being stuck inside the Amaterasu must be very boring. So he talks to it and learns to decipher the spikes in its bloodlust as if it were communicating. He eventually learns that it is a her. He learns that the rage she gives off feels a bit more like amusement when he shit-talks the sun church. He learns she's lonely and probably unaware of that. So he keeps coming. Week after week, year after year. Sitting on top of Amaterasu, he talks to her. He tells her about what's happening in Asakusa, the way the twins are growing so fast, how he's worried about his future responsibilities, how the granny down the street is teaching him how to cook so he can surprise Konro for his birthday, how it's been hot today but she probably feels hot all the time so he describes how cold feels like because she's probably forgotten, he tells her about the sunrise seen from Amaterasu, he asks her yes or no questions since it's the only answers he can really decipher. And he asks if she can remember what a hug feels like and she says no so he tells her, he's not the greatest with words but he tries.
She's a friend, at some point. He doesn't know If she sees him as a friend too, if she still knows how to feel positive emotions, but he figures the way her rage softens when she hears him coming says enough.