Li Xin, formerly known as Xin, is the main protagonist of Kingdom. He is the leader of the powerfully famous specialist army unit, the Hi Shin Unit, and a General of the Qin Military. He pursues the dream of becoming the Greatest General underneath the Heavens, continued on from the dream shared with his late brother-like friend, Li Piao, who was the one to inspire Xin to take up the sword and have their names etched in history through the battlefield.

Originally an orphan-turned-slave in the small Qin countryside village of Jouto, Xin left the village after witnessing Piao's death and met Ying Zheng, the 31st king of the state of Qin and He Liao Diao, a lookout for the lawless village near where Ying Zheng hid from enemies, as they became embroiled in the young king's battle for his throne during his half brother's rebellion.

As a military commander, Xin has fiercely participated in numerous campaigns and countless battlefields throughout the Warring States, having begun his career as a simple infantry soldier fighting under the famous late Duke Hyou in the Battle at Dakan Plains against the Wei Military, and eventually clashing with legends across the generations of military commanders of the continent, like the famed Rin Ko of Ren Pa's Four Heavenly Kings, Ri Boku and Hou Ken of Zhao's Three Great Heavens, and even bigger entities like the Ryo Fui Faction or the Coalition Army.

Despite his youth and relative inexperience, Xin has repeatedly clashed with formidable military opponents and secured victories across numerous battlefronts. Rising to the rank of 100-Man Commander after his first campaign, he distinguished himself by eliminating key figures in the Go Kei Army and played a crucial role in the Keiyou Campaign.

He later assisted Great General Ou Ki in the Battle of Bayou, which was commanded by two of Zhao's Three Great Heavens, and became a key figure in the successful invasion of Sanyou, where he directly contributed to Qin's victory over the legendary Ren Pa and his army. Under Duke Hyou, he fought in the Battle of Kankoku Pass, playing a vital role in key developments against the Zhao armies and personally slaying the sadistic Man Goku. Xin was later entrusted with the role of commander in the Battle of Sai, where he faced the Bushin Hou Ken, a foe he would ultimately defeat and kill on the final day of the Battle at Shukai Plains years later.

Xin has gained fame across the Warring States through his accomplishments, martial prowess, being one of the strongest combatants of his generation's military commanders, contributions to the Ei Sei Faction, and desire to Unify China as well as the focused hostility of Ri Boku in particular.