My One-Hit Kill Sister

 The My One-Hit Kill Sister.

  While saving a child from harm, otaku Asahi Ikusaba is transported to another world and believes he has reincarnated with cheat skills similar to those of the characters from his favorite games and novels. Unfortunately, reality strikes when an encounter with a powerful monster shows that he is still nothing more than an average person. In this moment of extreme danger, Asahi instinctively calls out to his overly doting sister, Maya, who appears out of nowhere, possessing the overpowered abilities he thought he had. Maya is ecstatic to see her brother again and resolves to use her newfound powers to protect Asahi no matter what. Together, the close-knit siblings attempt to find a way back home while enjoying their new lives. However, it soon becomes clear that their arrival in this world may have had a purpose. With the Demon King's overwhelmingly powerful army approaching, no one but Maya seems to be able to stand in its way.