In Honkai Impact 3rd, the younger sister is actually a Taimanin

Mizuki You was hit by a truck during a job interview. When he woke up, he found that he had traveled to a parallel world and turned into a child. He was picked up by a plump, mature and sexy older sister and taken home.
I originally thought that I could live the comfortable life I always dreamed of, but I found that something seemed to be wrong with this world!
The mature older sister who took him in is called Mizuki Shiranui, a Taima Ninja. Shiranui's daughter, his non-blood-related sister, is called Mizuki Yukikaze. The new lady who moved next door, Mrs. Shendong, is a snow woman. And a classmate who was on good terms with him suddenly turned into a succubus?
Someone in the school is holding a mobile phone and is ready to use software to hypnotize all the girls. The female knights of the demon world are trying to seduce them to join the demon clan. In this somewhat broken world, Shui Chengyou just wants to protect the people around him and keep them from being coveted by various yellow-haired people.
But they asked why everyone was relying on them?