He was the first one after Prophet Muhammad SAW who accepted Islam & the one who lead Muslims after Prophet Muhammad SAW.
How much did he know the Prophet (pbuh)?
May Allah be pleased with him, his sincere love to the Messenger of Allah (pbuh) and excellence in following his guidance.
What is special about him?
Allah the Almighty has praised him saying, "and the one who has brought the truth and [they who] believed in it - those are the righteous." (Az-Zumar: 33).