I Reincarnated as the Villainess’ Childhood Friend Who Falls into Darkness. If I Die, It’s a Guaranteed Bad End, So I Became the Strongest… but Hasn’t She Already Fallen into Darkness [and Turned Yandere]?

“If you were to disappear… I might just lose myself.”

Arcs is reborn into the world of the wildly popular otome game Celestia Kingdom. He thought he was just a background character—until it’s revealed he’s actually the childhood friend of the final boss! Determined to avoid the bad ending, he throws himself into his daily life as her servant. But it seems the true cause of her descent into darkness is… her childhood friend!?

An unpredictable tale of master and servant, between a slightly (maybe more than slightly?) yandere heroine and an obliviously overpowered protagonist, begins…!

https://www.novelupdates.com/series/i-was-reincarnated-as-a-childhood-friend-of-the-last-boss-who-has-fallen-into-the-darkness