Ojou-sama wa Sunao ni Narenai

Suichi seems like a normal student, but he's actually an experienced bodyguard. One day, he was asked by his mother, who is also a bodyguard, so that he transferred to the Reich Academy to help her protect a certain girl in school. The Reich Academy was once a famous women's school, and even after it became a joint school, it is still mostly full of girls. Moreover, for some reason he was surrounded by eccentric girls. One of them was a girl whom he had to protect, and she smugly said:
"From now on you are my" schwester "(sister)!"
At first the girls seemed impregnable, but he began to see their other side while getting closer to them.