Reincarnated Into a Game as the Hero's Friend: Running the Kingdom Behind the Scenes (Manga) Vol. 1

A modern man is reincarnated into the world of a fantasy game he knows well: as Werner, the son of a powerful noble. The thing is, Werner isn't the hero or even the villain - he's a background character likely to die during a climactic siege! Werner seeks out and befriends the hero, but he doesn't have powerful enough skills to be a hero himself. Unwilling to die as a later plot point, Werner uses his knowledge of the game and his power as a noble to aid the hero and defend against evil attacks.

That's when he realises that when he does so, he saves other people who were doomed as he was, which changes the 'plot' he thought was inevitable. As the hero's close comrade who can see the future, perhaps Werner will be a savior to this place, after all!