Karna is probably the only one in the Kaurav side of Mahabharat, who is considered to be a honourable man (no reference to Brutus intended).
Despite being a Duryodhan loyalist who called Draupadi a whore for sleeping with five men and who turned a blind eye to hajaar misdeeds, he is still considered a hero for not only having broken the fate of birth in a lower caste but also being the perfect friend and a man of honour.
Kunti got her boon from Durbasha (a sage known for his mercurial flitting between wrath and generosity) that she would be able to call any of the gods to sire a son for her. Now, why would this boon be useful to any woman married to an able-bodied king is a little beyond me. But given that Kunti's eventual husband (Pandu) was unable have sex (long story, kept for some other day), the boon became immensely handy.
Now there is a little doubt in the actual process of conception. Was it an immaculate conception, with the gods blessing Kunti with their sons? Or did it involve getting down and dirty (ahem) with them? In order to save myself from the wrath of Bajrang Dal activists, I will quickly progress.
So, right after getting the boon, Kunti summoned Surya and was blessed with a son, resplendent in the glory of the Sun god and factory-fitted with an armour (rendering him invincible). The unwed mother - obviously not having Neena Gupta as role model - abandoned the baby, who was picked up by Dhritarashtra's charioteer - a low-caste (suta) by the name of Adhirath.
All through his life, Karna was humiliated for his low-caste parentage (most often by the Paandavs) and his undying loyalty to Duryodhan was the result of the latter's hand of friendship. Of course, the Kaurav scion identified Karna to be a worthy adversary of Arjun and bought over Karna with his generosity.
In fact, the first time it happened when Arjun gave a spectacular demonstration of archery and challenged any one in the amphitheatre to a duel. In the crowd of cowering princes, Karna stood tall and accepted the challenge. He was asked by Kripacharya (Drona's brother-in-law and co-teacher of the princes) to name his lineage and kingdom since only princes were allowed to fight princes. Seeing the penniless Karna remain silent, Duryodhan immediately handed over the kingdom of Anga (present day Bhagalpur - bloody cheapster!) to Karna and tried to establish his royal status.
From here on, Karna was bound to Duryodhan in gratitude.
Though his 'royal' status did not wipe out his 'low-caste' background. The highest impact came during Draupadi's swayamvar, which was an archery contest designed by Krishna to ensure only Arjun would win. Karna tried his hand before Arjun, strung the bow and took aim perfectly before Draupadi spoke up and refused to marry a 'suta-putra'. This incident probably cemented Karna's hatred for Arjun even more.
Karna was surprisingly unlucky as well.
He was refused to be taught by Drona because of his low caste. The only other teacher of the martial arts was Parashuram, but he imparted his knowledge only to Brahmins. Karna pretended to be a Brahmin and enrolled with him. After the completion of a successful education, Parashuram fell asleep on Karna's lap and some insect bit Karna's thigh drawing blood in the process. Despite the excruciating pain, Karna did not even budge so that his teacher is not disturbed. When Parashuram woke up, he saw the blood and deduced that this ability to withstand pain could hardly be a Brahmin's forte. He called him a 'deceitful Kshatriya' and cursed him that he would forget all the teachings at the time he needed them most.
And the ill luck did not end here.
On his way back from the ashram, Karna mistook a cow for a deer and killed it. The brahmin who owned the cow, cursed him that his chariot would be rendered immobile when he would be fighting for his life.
So, Karna went into the battlefield of Kurukshetra knowing fully well that he would be without the use of his biggest strengths when he faced his strongest foe. Despite this knowledge, he gave himself a chance against the greatest archer of all times and took up arms for his friend.
Apart from bad luck, the pressure on him to make him fail against Arjun was immense. The elders like Bhishma and Drona openly backed their favourite against this low-caste outsider.
At the beginning of the battle, Bhishma realised that himself, Drona and Karna fighting together would be a force which the Paandavs would not be able to match. So, when he grouped the Kaurav army under the maharathis, Karna was not among them. When Karna asked him, he called him an ardha-rath (half a rathi) and said he did not deserve to command any of the battalions. Thus insulted, Karna refused to fight under Bhishma and took up arms only after Bhishma had fallen.
When Bhishma and Drona had both fallen, Karna was appointed the commander and he requested for Shalya to be his charioteer. Shalya was the uncle of the Paandavs and was known to be a charioteer par excellence. But when Shalya took over the reins, he actually did everything to demoralise Karna and undermine his confidence. His love for the Paandavs was so much that he tried his best to reduce Karna's capabilities on the battleground.
Taking on the greatest archer in the world with your own side opposing you is quite a task.
As a man of honour, he stood unparalleled (to the point of being stupid at worst and impractical at best).
His kabach-kundal (armour-earring) was a gift from his father (Surya) and made him invincible to mortal hands. Arjun's father (Indra) knew his son would not be able to breach this armour and decided to intervene.
Karna had a vow that when he did his surya-pranaam at dawn, he would never refuse anyone. Indra took this opportunity and appeared one morning before the war in the guise of an old beggar. And asked Karna to give him his armour.
Karna recognised the beggar to be Indra since no beggar would have any use for the armour. Despite seeing through the ploy to save Arjun, he handed over the armour. He also took the opportunity to ask Indra for the lethal Indraastra, a weapon which had no equal. He did this because he knew his earlier weapons would be forgotten when he engaged in the final showdown with Arjun.
Unfortunately for him, he could not preserve the weapon (one-time use only!) for Arjun. On the night when Jayadrath was killed, the two sides agreed to continue the battle after nightfall. A decision instigated by Krishna (to which Duryodhan agreed), it turned out to be a nightmare for the Kauravs. Because Ghatotkoch (half-rakshas son of Bheem) ascended to his true powers and ran roughshod over the Kaurav army. It seemed that he would finish off the battle in that one night and all weapons were ineffective on him. Duryodhan himself was almost killed by him. (Ghatotkoch spared him because of his father's vow to kill all the 100 brothers!). The only weapon that could have killed Ghatotkach was the Indraastra, which Karna was preserving for Arjun.
Duryodhan taunted Karna by saying that when the Kaurav camp would become barren, he would still be chasing Arjun with the weapon in his hand. Unable to see his friend's plight, Karna used the Indraastra to kill Ghatotkach. The weapon returned to Indra after its one use and Ghatotkach expanded himself several times his size before dying and crushed thousands of Kaurav soldiers to death! At this point, Krishna got down from his chariot and danced in joy. He told a grieving Bheem that Ghatotkach had done his biggest service by taking the blow that could have killed Arjun. Having seniors continue in the team at the cost of juniors is not a phenomenon restricted to BCCI alone!
On the eve of the battle, Kunti secretly met Karna and revealed his parentage. She offered to tell the world that Karna was the original Kaunteya (son of Kunti) and as the eldest Paandav, he had the rights to the throne of Hastinapur.
Karna told her that she had only one opportunity to reveal his parentage and that was when his challenge to fight Arjun was negated on grounds of low caste. That was the time she could have been courageous and won him over. Now, she was trying to entice him because she feared that the lives of her five sons were at threat. He assured her that he would not kill any of the brothers except Arjun and at the end of the battle, there would still be five Kaunteyas. Either of Karna and Arjun would survive the battle.
At the end of the battle, the Paandavs performed the last rites of all the elders but they turned away from the body of Karna. At that time, Kunti revealed the secret to the brothers. This caused terrible grief among them. The younger four were probably thinking, "Thanks to mom, we blew the chance of having a real dude as Big Bro instead of that gambling wimp". Yudhishthir (God only knows what he was thinking) was angered enough by Kunti's secretive nature to curse her (yes, his own mom!) and all the women in the world (talk about the blame game!) that they would never be able to keep a secret.
From then on, men and women both gossip but women get blamed for spilling the beans!
In the pantheon of heroes, Karna stands tall. He was defeated by Arjun (though by slightly dubious means). But ultimately, his honour and valour became apparent to all through legends and today, the number of people named Karan are as many as those who are named Arjun.
Karna and Indrajit are the only two 'losing' warriors from the Indian epics who have children named after them. Bibhishan, on the other hand, was on the winning side but we still label traitors by his name.
What interests you - instant victory or everlasting glory?
ASIDE 1 (Books): In Shashi Tharoor's The Great Indian Novel, we have a character called Mohammad Ali Karna, who was born to Kunti after an affair with a Greek called Helios. He went on to form the Karnistan when Ganga-ji (a cross between Gandhi and Bhishma) refused to elevate him in the Kaurav Party.
ASIDE 2 (Films): In Shyam Benegal's Kalyug, Shashi Kapoor plays Karan Singh, who rose from humble backgrounds to become the mainstay of Dhanraj in his business rivalry with his cousins (Dharmraj, Balraj and Bharatraj). He was shot by a supari killer, when he stopped on a highway to change tyres.