Spoiler from raws.
It is Seno. He has been in love with Natsuka before they were fuckbuddies, in school. Natsuka didn't remember him when they meet again. Seno witnessed Natsuka and Hakushima talking about the book, that's why he has the same one and that's how he knew what to say to Natsuka to convince him... and like he said Natsuka wanted to believe he finally had Hakushima even though there were several hints that he was not Hakushima --- all the times Natsuka thought that something was unexpected or didn't match what he knew from the other man. The 'soul' wish is actually from Seno wishing Natsuka to love his personality, not his body or the idea that he is Hakushima.
Oh my god thank you! I like it resolution, cause I really like Seno he's somehow bitchy but adorable. Now I will wait and see how they life happily ever after(hopefully), and then I can rest in peace
I'm confused as fuck. I don't know if he is Seno and be him all the time or he is after all Hakushima and he's only testing if Natsuka love his soul. I need answers!!!