Who was Banquo in real life?

Shakespeare's historical source of the play
Macbeth was Raphael Holinshed's Chronicles of England
Scotland, and Ireland which was written in 1587. The book refers to Banquo as
an acquaintance of Macbeth's. According to the history, Macbeth, with the help of
others, conspired with Banquo to kill the weak and ineffectual King Duncan.

Now, as Macbeth was dedicated to King
James and Duncan and Banquo were, as legend had it, both in the royal line of King
James, Shakespeare changed the history and made the men sympathetic
characters.