Macbeth and Banquo are parallel characters because both are nobles, and both start out brave and honest.
They often appear together, until Macbeth kills Banquo. Macbeth and Banquo are both noblemen. When they first appear, they are together. The witches share the prophecies of both together. When Macbeth is to be king, Banquo’s sons are to be king. One’s fortune is the other’s. Macbeth is upset by this.
They hail'd him father to a line of kings:
Upon my head they placed a fruitless crown(65)
And put a barren sceptre in my gripe, (Act 3, Scene 1, p. 42)
Macbeth uses “Banquo’s issue” as an excuse to kill him, which is ironic because the two characters are so linked that this is one of the actions that ensures Banquo’s sons’ rule. It is one of the steps that leads to Macbeth’s downfall.
Even after Banquo is dead, he still appears. He comes to Macbeth’s banquet, causing Macbeth to get so upset that Lady Macbeth has to send the guests home.
The time has been,
That, when the brains were out, the man would die,(95)
And there an end; (Act 3, Scene 4, p. 51)
Banquo’s presence does not end there. He appears again when the witches make the last set of prophecies, this time holding a mirror showing his sons as future kings.