By the time he would know Vrish , the bond between him and Karna had already been created .
He initially decided to quickly grab him to neutralize Arjun's growing power
but later genuinely became his friend .
What interests me is the fact that Bheeshma told Karna that he would not tell the Pandavas of his identity but only to Duryodhan so that Duryodhan knows of his nobility .
Researchers [ some] say that he did tell Duryodhan and that led him to lose interest in winning and sit in that hiding spot listlessly .
When Yudhishtir called him out Duryodhan frankly said he had lost interest and he could have the kingdom .
Upon which Yudhishtir said he did NOT want the kingdom as a donation but by defeating him .
Duryodhan then came out and when asked to pick he picked the BEST in mace ...not the weakest ...Bheema .
Just as Draupadi did not use the third boon and showed character , i am afraid he did too .
And ultimately he was killed by breaking rules of a mace fight .
How many times Duryodhan himself unscruplously broke rules is a seperate argument ...this actually happened .
I am afraid he has come across as rather valiant in this fight .
Regarding him knowing . some researchers say Bheeshma told him and he cried inconsolably at Karna's nobility and steadfast determination not to desert him and completely lost interest in any win after that . By that time the war was lost too . But i am talking of his emotional condition when he sat in the cool spot underwater .
Personally i feel it would not shake his confidence in Karna coz he knew Karna too well .