Karna's Origin, Training, and Rivalry with Arjuna
Kunti learns through a spy that her abandoned firstborn son wears celestial armour. Adhiratha sends the boy to Varanasahrya to learn weapons, where he studies under Drona, Kripa, and Rama, becomes Duryodhana's friend, and locks into a lifelong rivalry with Arjuna that makes Yudhishthira despair.
She reared him appropriately and he grew up. He was brave. Since that time, she also gave birth to other sons. The boy was clad in expensive armour and wore golden earrings. On seeing this, the brahmanas gave him the name of Vasushena — "born with wealth."
Thus the infinitely valorous one came to be known as the son of a suta (charioteer) by the name of Vasushena. He was also called Vrisha. The suta's eldest son grew up powerful in his limbs.
Through a spy, Pritha — Kunti — got to know that he was clad in celestial armour. She had abandoned him at birth, placed him in a basket on the river, watched him float away. Now she knew: he lived. And he wore the armour she remembered.
In due course, when suta Adhiratha saw that his son had grown up, he sent him to the city of Varanasahrya. There, the boy went to Drona to learn about weapons. The brave one became Duryodhana's friend. Having obtained the four kinds of weapons from Drona, Kripa and Rama — Parashurama himself, the Brahmin-warrior who had once wiped out the entire kshatriya (warrior) caste — he became famous in the world as a great archer.
Having allied himself with Dhritarashtra's son, he was engaged in enmity towards the Parthas — the sons of Pritha, his own brothers, though he did not know it. He always desired to fight with the great-souled Phalguna — Arjuna.
Ever since they first saw each other, he competed with Arjuna and Arjuna competed with Karna. On seeing that he possessed earrings and was clad in armour, Yudhishthira thought that he was invincible in battle and was miserable.
At noon, Karna used to worship the radiant sun, standing in the water and with his hands joined in salutation. Brahmanas who desired riches approached him then. At that time, there was nothing that he would refuse to brahmanas. Indra assumed the form of a brahmana and approached, saying, "Give me." "You are welcome," replied Radheya. Aranyaka Parva, Chapter 590