I Became the Demon King’s Right Hand so I Will Alter the Original Story

Tooru Kariya, a corporate slave of an OL, finds out that her only solace, her favorite character, the "Demon Lord" in the manga "Lapis Lazuli World", has been killed. In despair, Tooru falls off a building. But she wakes up to find herself in front of the Demon Lord's castle! Toru has transmigrated into Lapis Lazuli World and unexpectedly befriends the Demon Lord, so she offers to serve him.

There is only one thing to do! I will make sure the Demon Lord has a happy ending!

(Source: B's-Log Bunko, translated)

Note: includes two prologues, two epilogues and two extra chapters.