In 2606, when Senbei was three years old, he was attracted by the sword in his family shrine as he felt he could hear his ancestors' calling for his name. His father promised that Senbei would own the sword, which had been passed by generations for 1000 years, once Senbei finished his first battle someday.
He began to learn the art of sword since he was three and demonstrated his gifted talent. He become well-known when he was seven years old as he bested twenty-one adult samurai from the elite force. When he was nine years old, Senbei was recognized as an "adult" before he participated in his very own first battle and inherited the family's sword as his father had promised. Unfortunately, Yagyu's clan was almost destroyed during the battle against the Empire's invasion to the main island of Japan, leaving Senbei as the only survivor in the winter of 2612.
As the enemy forces occupied Kyoto, the nine years old Senbei carried the sword and his younger sister to break the siege. He then began his vagrant life until he found his place in a Resistance's camp.
However, a few years later, he witnessed his six years old sister being killed by his own mother, who had become a zombie during a siege by zombie corps. Senbei was later saved by a group of mercenaries. He then joined the group and fought against the Empire until its fall. However, even after the Empire had fallen, the zombie corps remained. He swore to fight against the zombies for revenge. One day, he found his zombified sister among other zombies. He killed her out of penance.