500-nen No Itonami Dj - 500-nen-go
This is included in "500-nen No Itonami" on this site, but this is the happy ending for them.
Void (zariya Ranmaru)
Seven years after his lover Ren died, Maki is still angry at him for his betrayal, because while Maki had been completely devoted to Ren, Ren had used him as a substitute for his straight, married brother. When Maki's brother gives him a gift, a "pet" model robot called Arata designed after the late Ren that even has some of his memories, Maki takes the chance to vent the anger that has kept him trapped in the past. But despite looking like Ren, Arata is nothing like him, and after two weeks of cruel treatment, Maki's anger turns to loneliness when he remembers Arata's affection for him is nothing more than imprinting. Original comment: I think this may have influenced a D/C AI fic I've read, the beginning especially? It's eerily similar. [On researching, I believe the fic predates the manga so it must've been a coincidence.] 2022 ETA: Purchased from SuBLime
500-nen No Itonami