Yamiochi Last Boss Reijou no Osananajimi ni Tensei Shita. Ore ga Shindara Bad End Kakutei nanode Saikyou ni Natta kedo, Mou Yamiochi [Yandere-ka] Shitemasen ka?

A character named Arcs is reincarnated into the setting of the popular otome game Celestia Kingdom. Initially believing he is a minor character, he learns he is the childhood friend of the final boss. To prevent a bad ending, he dedicates himself to living as her servant. However, it appears that the reason the final boss turns to darkness may be linked to him. A story unfolds involving a heroine who may be yandere and a protagonist who is unaware of his power.