Maou no Migiude ni Natta no de Gensaku Kaiaku Shimasu

Toru 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, Toru 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)