Undertale: I Forge the Perfect Ending!

(Potential deal-breakers: There is a female lead; neither lead is particularly powerful; there are other transmigrators; some details involving time travel aren't entirely consistent; and the early pacing is slow. If you're fine with all that, read on!)

AU Name: LD! Undertale

Lin De unexpectedly finds himself transported into the world of UT!

To achieve an even more perfect ending, he journeys through time and space, striving to escape the grasp of the unseen hands pulling the strings and uncover the truth of this world...

"Huh?! Wait! No! How did I fall to my death right at the start?!"

Well, that's where this story begins.

"I'll give it everything I've got to make sure this world keeps running the way it should!"