Kenshiro was originally an orphan from Taiseiden, Shura and a descendant of the Hokuto Sōke bloodline. He was sent to the mainland by Jukei as a baby to be adopted by Ryuken, master of Hokuto Shinken. Ryuken named him after his older brother Kenshirō Kasumi since they shared the same Big Dipper-shaped birthmark on their heads.
Kenshiro watched his adoptive brothers train in the art of Hokuto Shinken and decided to join them in the fight to become Ryuken's successor. Despite being younger and more naive than his older brothers, Kenshiro was a promising student who ultimately earned the title of the successor.
After the nuclear war of 199X and the death of Ryuken, Kenshiro left with his fiancée, Yuria, to find a future for themselves in the post-apocalyptic world. However, this was curtailed by Shin, a rival from the Nanto Koshū Ken school, who defeated Kenshiro and engraved the trademark seven scars on his chest, before kidnapping Yuria for himself.
Kenshiro survived his ordeal and spent the next year wandering the wasteland for revenge. During this time, he developed the killer instinct needed to survive in the war-torn age and befriended two young orphans, Bat and Lin, who followed him on his journey. He discovered that Shin had assumed the title of 'King' and was the leader of the eponymous KING organization. After defeating Shin's four generals ('Four Jacks' in the anime), he infiltrated his capital city, Southern Cross, to settle things once and for all. Driven by his new strength, Kenshiro soundly defeated Shin but he was too late to rescue Yuria. Before his death, Shin revealed that Yuria had already committed suicide to stop his violent ambition. Refusing to die from Kenshiro's technique, Shin leaped to his death from his palace balcony, as Yuria did before him.
Kenshiro continued to journey the wasteland, never staying at one place for long, as the curse of Hokuto leaves only chaos in its wake. He made a career of destroying those who preyed on the weak and innocent, such as Golan, Jackal and the Fang Clan, and built a reputation as a 'savior' among the people. He also discovered that his three older brothers and fellow Hokuto Shinken practitioners: Jagi, Toki and Raoh, had all survived the nuclear holocaust. Jagi, the second-youngest, had long hated Kenshiro and was now committing acts of carnage in his name. It was also revealed that Jagi manipulated Shin into taking Yuria, as well as kidnapped Rei's sister, Airi, and murdered their parents. Kenshiro caught up with Jagi and subjected him to a gruesome death. Kenshiro then fought against his second-oldest brother, Toki, who had succumbed to madness after the apocalypse, but this was in fact Amiba, a jealous rival of Toki, who was impersonating him. The real Toki was imprisoned at Cassandra by Raoh, the eldest brother, who had taken the title Ken-Oh (拳王 King of Fists) and aimed to bring order to the world by any means necessary. With the help of Rei and Mamiya, Kenshiro toppled the great prison and was reunited with Toki. He would go on to become a valuable ally to Kenshiro in his journey and intervened during his first fight with Raoh, as they were on the verge of killing one another.
Kenshiro joined forces with Shu, leader of a guerilla outfit against Souther, the Holy Emperor, and master of Nanto Hōō Ken, who enslaved children to build his empire. Kenshiro was defeated by Souther when his vital point attacks had no effect on Souther's body. On the brink of death, Kenshiro was rescued by Shu's son, Shiva, who gave his life to aid his escape. However, in retaliation, Southern stormed the rebel hideout and had Shu executed at the summit of the Holy Pyramid. Kenshiro recovered from his wounds and this time he uncovered Souther's secret: the position of his heart and his vital points were reversed. Kenshiro took pity on Souther's tragic past and killed him with a merciful technique, bringing an end to the Holy Emperor's reign.
With the final battle with Raoh drawing ever closer, the mysterious Last Nanto General emerged to challenge Raoh's conquest. It turned out that the Nanto General was none other than Yuria, Kenshiro's betrothed. In actuality, she had been saved by the Nanto Goshasei after attempting suicide at Shin's palace and her survival was kept a secret to protect her from Raoh. Kenshiro fights Raoh using the ultimate Hokuto Shinken technique Musou Tensei, drawing on his profound sadness to make Raoh tremble in fear. However, Raoh kidnaps Yuria during his escape and prepares to kill the woman he loves if it will imbue him with the same sorrow. Kenshiro and Raoh have their final battle at the Hokuto Renkitōza, with Raoh having killed Yuria to unlock Musou Tensei. Kenshiro's greater understanding of life and death gives him the edge over his older brother and leads him to victory. Yuria awakens after the battle, as Raoh had actually put her in a false state of death when he realized she was terminally ill. After Raoh's death, Kenshiro spends his next few years living in peace with Yuria, whose disease had been subdued by Raoh's power.
The latter half of the manga (and the corresponding Hokuto no Ken 2 anime adaptation) involves Kenshiro joining forces with the grown-up Bat and Lin, who have formed the Hokuto Army to oppose the tyrannical Jakou and his Imperial forces. The story eventually takes Kenshiro to his birthplace, the Land of Shura, where he learns of his heritage and fights against the warlords who oppress the land. This includes his estranged blood-brother, Hyoh, and the nefarious Kaioh, Raoh's other blood-brother. The final story arcs of the manga (which were not adapted for the TV series) see Kenshiro taking Raoh's orphaned son, Ryu, under his wing before continuing on his own where he settles the love triangle between himself, Bat and Lin. In the end, Kenshiro heads into the desert alone to fulfill his destiny as the Hokuto Shinken successor.
It's assumed that he eventually comes back to Ryu and trains him to become the next Hokuto Shinken successor.