Dear (future) Husband

How to marry a fantasy hero?
The Autumn Leaves Have Fallen On Glory's Path by DemonMaster666 was a webtoon soaring high above others, and reader FrozenMoonlight its most loyal fan.
So loyal, that after his untimely death, he finds himself transported into its world with a mission to make sure the dashing hero ends up happily married to his best friend.
The only problem?
The role he's been assigned to is that of the best friend.