Eroge no Heroine wo Netoru Otoko ni Tensei shita ga, Ore wa Zettai ni Netoranai

When I came to my senses, I was reborn in the world of eroge. I am not Osamu Sasaki, the main character of the eroge "Boku wa Subete wo Ubawareta." I was reincarnated as his best friend, Towa Yukishiro, who cuckolds Ayana Otonashi, a childhood friend and heroine who is devoted to Osamu. Fortunately, I have a year to spare before the game story begins. I have no taste for cuckoldry myself, and if I didn't take her away from him, they would be happily married... "It's just the two of us... isn't it?" As soon as Osamu was gone, Ayana, who had kept her body in close contact with him, climbed onto my lap―could it be that the two of us were already in a relationship before the game even started!? Is this NTR? BSS (I liked her first)? Neither, this is a rich pure love story! (Source: Kadokawa, translated)