Lyren reincarnated as the NPC of a light novel.
Using his knowledge from the original work as his "Future Sight," he stole the original main character's achievements by anticipating the light novel's events.
But after reaching the end of the original narrative, and no longer able to rely on his knowledge of future events, the glamorous life he built up crumbles apart.

He becomes riddled with debt, loses his fiancée, and his life turns to shambles.
That's when the rightful protagonist, Albert, appears and...

Such is the setting of this Original Main Character x Substituted Background Character story.
This story features an obsessive seme (top), an uke (bottom) with an inferiority complex toward said seme, and a happy ending only for the seme, so... please be advised if that is not to your liking.