When Haruka Sakura arrives at Furin High to fight his way to the top, he doesn't come alone. Following half a step behind is his twin sister, Lydia—a silent girl with honey-golden eyes and a power that secretly eclipses even Bofurin’s legendary leader, Hajime Umemiya. As the twins integrate into Class 1-1, Lydia acts as the "Hidden Sword," quietly neutralizing threats while Sakura grows from a lonely outcast into a true leader. From the brutal showdowns with Shishitoren to the desperate swarm of the Gravel gang, Lydia ensures her brother’s path stays clear. However, as the Noroshi War looms, her secret strength becomes Bofurin's final line of defense against the "monsters" of the past.