"Yukino Yukinoshita is just a little sister! I'm the older brother!"
"Yukihira Yukinoshita is the little brother! I'm the older sister!"
"…"
Yukihira and Yukino had been at odds since before they were born. They fought over who would be born first, nearly killing Mrs. Yukinoshita in the process. When the doctor finally performed a C-section, their umbilical cords were still tangled together.
Their ill-fated bond lasted until Yukihira was in elementary school. That year, he was kidnapped and taken to the war-torn Middle East. After being baptized in blood and fire, he was adopted by a red-haired woman before he could turn to the dark side, and joined a certain secret organization.
Seeing Yukihira grow more and more withdrawn, the red-haired woman assigned him a fake mission: infiltrate a high school in Japan as a student and stop the terrorist plot of an evil organization.
So, in order to fool everyone, he began posing as an ordinary high school student.
All in all, this is probably a story about a tsundere big brother who's obsessed with his little sister.
