Who was Umar Ibn Al-Khattab?
He was the one who wanted to kill Prophet Muhammad SAW but Allah SWT guided him and he accepted Islam.
How much did he know the Prophet (pbuh)?
He was the one who, before accepting Islam, used tell Mushrikeen Makkah that Prophet Muhammad (pbuh) always speak truth.
What is special about him?
Allah the Almighty has accept duaa of Prophet Muhammad SAW in favor of him. And he accepted Islam.