Complex sentence: Katsushirō, the youngest samurai, begins a relationship with Shino, who had been forced to masquerade as a boy by her father to protect her from the supposedly lustful samurai.
Split into simpler sentences: Katsushirō, the youngest samurai, begins a relationship with Shino. Shino had been forced to masquerade as a boy by her father to protect her from the supposedly lustful samurai.