For his entire life, Karna lived believing that he was the son of a charioteer. Even though Duryodhana made him a king, no one around him ever let Karna forget his humble roots. The severest and the most amount of ridicule came from the Pandavas who, like Karna, didn’t know of his true origins.
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Throughout the years, Kunti kept mum about Karna, even though she knew who he was when he made his debut in the court soon after he had completed his training under Drona. Bhishma too knew about Karna’s origins, likely through his mother, the river Ganga where the latter was abandoned as an infant. Krishna, being a god and omniscient, also knew. As did Sahadeva, who on the passing of his father Pandu received a divine vision that let him see everything that had passed and everything that would come to pass.
While Sahadeva couldn’t reveal the truth to Karna since he’d made Krishna a promise to never reveal anything to anyone unless they asked for it specifically, all the others revealed to Karna the story of his birth. However, given the timing that Krishna, Kunti, and Bhishma chose to reveal the truth to him – just before or during the war – Karna figured that it was just a ploy to get him to switch sides.
However, Karna had sworn loyalty to Duryodhana and opted to fight by his friend’s side. Not only did he go into the battle despite being cursed to die on the field, Karna did not reveal the truth to his friend, Duryodhana.
In fact the one person who may have truly appreciated Karna’s loyalty was the one person who never learnt the truth. The story of Karna’s origins became public knowledge only after the war when the dead were being cremated. Kunti reveals the story to her sons and then the tale spreads.
By this time Duryodhana had already lost his fight against Bhima and had died and no one around him at the time had the knowledge of Karna’s birth. The truth of Karna’s birth, thus, becomes public only after Duryodhana’s death and the Kaurava prince, the man who was his closest friend.
Duryodhana dies believing that he had made a charioteer’s son a king whereas the truth was that Karna chose to give up the chance to rule the entire empire just so he could fight by his friend’s side.