Kars is the primary antagonist of the second story arc of the series and serves as the ambitious and ruthless leader of the Pillar Men, an ancient and powerful race of humanoid beings. Believed to have been born around 100,000 BC, Kars is exceptionally intelligent and driven by a singular desire to conquer his race's greatest weakness, the sun, in order to become the ultimate life form. His name is derived from the 1980s new wave band The Cars.

In terms of personality, Kars is defined by his cold, calculating nature and his single-minded dedication to his goals. He holds a deep-seated apathy toward humanity, viewing humans as insignificant creatures unworthy of consideration and often killing them without hesitation. Despite this general disdain for people, Kars exhibits a surprising and notable sense of compassion for other forms of life. On separate occasions, he has demonstrated a willingness to protect animals, such as slicing a car in half to save a puppy from being run over and carefully maneuvering his body during a fall to avoid crushing a patch of flowers. This selective empathy creates a distinct contrast to his otherwise ruthless demeanor, though it does not extend to his treatment of humans who stand in his way. He is also shown to be loyal to his own kind, expressing genuine anger and sadness following the deaths of his fellow Pillar Men, Esidisi and Wamuu, and seeking to avenge them even after achieving his ultimate form.

Kars's role in the story is that of the main villain driving the conflict. His primary motivation is to obtain the Red Stone of Aja, a gem that would perfect the Stone Mask and grant him the power to control his own biology completely, thereby making him impervious to sunlight and capable of mastering every ability of every living creature. His background reveals that he created the Stone Masks originally as a means for the Pillar Men to evolve, but his methods were deemed so extreme by his own people that he, along with his comrades, was banished and forced into a millennia-long slumber before being awakened in the 20th century. His quest puts him into direct conflict with Joseph Joestar, the protagonist, as well as with the Nazi scientist Rudolf von Stroheim, whose forces are also seeking to exploit the Pillar Men's power.

Throughout the narrative, Kars undergoes a significant development, culminating in his transformation into the Ultimate Life Form. By using the Red Stone of Aja on a specially crafted Stone Mask, he successfully overcomes the sun and gains godlike power. As the Ultimate Life Form, his intelligence and physical capabilities are magnified to an omniscient level. He gains the ability to instantly adapt to any environment, transform his body into any form of life, and replicate the unique biological traits of any creature, from growing wings for flight to generating sharp shells as armor. One of his most notable base abilities is Light Mode, which allows him to create razor-sharp blades from his arms that can reflect blinding light to disorient and slice enemies. In his ultimate state, his powers become near-limitless, including transforming his own hand into a living squirrel to attack others or generating a school of piranhas from his body to devour opponents from within a plane. While his intellect makes him a formidable strategist capable of using any tactic to win, his overconfidence and the bizarre circumstances of his final battle ultimately lead to his defeat when he is inadvertently launched into outer space by a volcanic eruption, where he freezes and is left unable to die, stuck in an eternal state of ceasing to think.