As a samurai Sato served the Lord of Clan Makoto. He was a dedicated and highly skilled warrior, but his commitment to an honourable life was probably his over-riding motive.
When Lord Makoto was killed by what appeared to be demons bearing the crest of the rival Clan Rikushira, Sato pledged himself to avenge his master and then commit suicide (feeling he had failed by allowing him to be killed). On the way to Castle Rikushira, he encountered Izzy, and with her help was able to enter the fortress.
Lord Rikushira had entered into an alliance with aliens known as the Gaijin. However, the vision of Japan's future that Rikushira's mother Asami was able to extract from Izzy's mind drove her insane and she used the alien technology the Gaijin had provided (source of the demons that had killed Makoto) to destroy the castle, planning to launch a pre-emptive attack on the West to prevent the atom bombings of the 20th century.
The Eighth Doctor persuaded the Gaijin to stop Asami, and in the chaos Sato was able to kill Rikushira. He was mortally wounded, however, and would have died had the Doctor not treated him with life-preserving nano-drones the Gaijin had supplied Rikushira with as part of their deal.
However, to his horror, Sato discovered that the nano-drones could not be deactivated and his attempts to commit honourable suicide, now that his mission was fulfilled, were futile. He was effectively immortal and invincible, but at the same time had lost the thing that mattered most to him: his honour. For Izzy's sake he did not kill the Doctor, but left the time travellers, an angry, bitter man. (COMIC: The Road to Hell)
Immortal servant of the Master
Sato became a pirate but tired of this and enabled himself to be captured. The Spanish Inquisition were unable to kill him, so imprisoned him in Saragossa for 50 years. Sato was granted the chance to take revenge on the Doctor when he was contacted by the Master. The renegade became a mentor to Sato as part of his plan to take control of The Glory.
At the Master's behest, Sato adopted the identity of Cardinal Morningstar, leader of the Church of the Glorious Dead. The Church launched a relentless holy war which perverted the natural course of Earth history, Sato becoming absolute ruler at the beginning of the 19th century, and led to the planet becoming known as Dhakan. By the 20th century Earth had mastered interstellar travel.
To weaken the Doctor's resolve prior to their battle for the Glory, the Master had Sato destroy the civilisation of the planet Paradost with the help of Ash Wraiths. The Doctor's companions Izzy and Kroton eventually destroyed the Wraiths, but Sato was able to shoot Izzy with a TCE and take her captive.
With his prisoner (and, unbeknownst to him, Kroton aboard his ship as a stowaway), Sato returned to Dhakan, where he believed the Master's final victory over the Doctor was imminent. However, the battle for the Glory was not between the Doctor and the Master, as everyone had assumed, but between Sato and Kroton. Sato's warrior skills and immunity meant he outmatched even the power of a Cyberman, but Izzy had managed to free herself and forced Sato to take a mnemonic crystal, confronting him with all the suffering he had caused and breaking his will. Kroton, however, was able to take the crystal and confront his memories, rendering him the victor of the duel. With this, Kroton was placed as the overseer of the Glory and granted Sato the honourable death that he had always wanted, as well as repairing the damage Sato had done to Earth's history. (COMIC: The Glorious Dead)