Kiem Sun was born during the Sung Dynasty in China. In 1276, He died during a battle against invading Mongols under Kublai Khan. He was soon found by May-Ling Shen who taught about his Immortality and how to survive. She did, however, regard him as a bad student.
He would meet Duncan MacLeod and the two became friends. The two men meet an alchemist who told them about the Quanla Root which he believed will ensure the betterment of humanity. Kiem wanted to learn how to make the formula, and the alchemist taught him but left out the most important ingredient, believing that the Root would be dangerous in the wrong hands.
Kiem would relocate to holy ground where he would experiment with the Root. In 1780, Kiem was still experimenting with the Quanla Root. By that time, his friend Duncan MacLeod was visiting him. After exchanging pleasantries, Kiem announced that he had finally made a breakthrough with the Root and held a demonstration for Duncan. Kiem summoned two warriors and pit them against each other, after giving them the root potion. Duncan was horrified at Kiem’s abuse of the mortals. But he was even more horrified when the warriors fell to the ground and died due to the side effects of the drug. Outraged, Duncan argued with his friend and told him to stop experimenting with the drug. Later when Duncan was about to leave Kiem sent him to learn martial arts from his former mentor May-Ling Shen.
Kiem, however, continued to experiment with the root for many decades. By the 1990’s Kiem was living in Seacouver in the United States, residing in a Shaolin temple. His assistant, Chu Lin, whom he told about his Immortality, also knew about the root. When a man died exhibiting the same symptoms as the long ago warriors, Duncan went to see Kiem, demanding answers about what happened, Kiem denied having anything to do with the man's death and told MacLeod that his assistant, Lin stole the completed potion.
Duncan then fought a practice bout and tested Kiem Sun's sword skills, and found them wanting. He reluctantly agreed to join with Kiem Sun in finding Chu Lin and the potion. Duncan and Kiem Sun arrived at a garage where Richie and his friend Angie are also trying to confront Chu Lin. Chu Lin attacked Duncan, but the Highlander proves more than a match for him, and disarmed him. Duncan demanded that Chu Lin surrender himself to the police and make a full confession, but Kiem Sun drew his own blade, and cut down Chu Lin in cold blood, then turned on Duncan.
Displaying hidden prowess, Kiem Sun challenged his former friend, fighting MacLeod until wounded. Duncan defeated Kiem, but rather than take his old friend's head, Duncan poured out the only remaining portion of Quanla potion, much to Kiem Sun's dismay.
He revealed that he knew Kiem Sun's reasons for perfecting the potion: to obtain personal guards to fight against Immortal opponents in the Game. Duncan informed Kiem Sun that their friendship was now ended, that he had crossed the line, and the next time MacLeod saw him, he would kill him. He left Kiem Sun to ponder, taking Richie and Angie with him.
Kiem later moved to San Francisco where he built a Buddhist temple overlooking the bay.