Banquo is all the things the witches predict:
Throughout Macbeth, Banquo is a foil for Macbeth. He is loyal to Duncan and a skeptic of the witches when Macbeth is neither. He is a devoted father, while Macbeth is obviously not (he has no children and will murder children). Because of his loyalty and knowledge of the Weird Sister, Banquo will become a threat to Macbeth when he is crowned.
After Macbeth kills Banquo but fails to kill Fleance, he will say:
Whereas Banquo is the serpent, Fleance is the worm that escapes. Banquo, then, will be the father of Kings, though not one himself, just as the witches predicted.
Banquo will become a doppelganger (a ghostly twin) after he is murdered. In the banquet scene, Banquo will haunt Macbeth; therefore, he is a revenge ghost. Banquo serves as a supernatural symbol of moral retribution in the play, causing Macbeth to feel guilt and mental illness.
In many ways Banquo is a mirror image of Macbeth. Indeed, early on, the two men are strikingly similar: both men are brave warriors; they are good friends; they are both promised things by the witches; they are both honored by the King. And it is because they are so similar that they are easily compared.
Where they differ serves well to define each man: where Banquo has a son, Macbeth has a wife: where Banquo is rather indifferent to the witches' prophecies, Macbeth is obsessed.
By Act Two, these differences in the two men will lead Macbeth to kill King Duncan, and in Act Three they will lead Macbeth to kill Banquo and try to kill Fleance, Banquo's son.
You can proceed on the basis of the following leads:
a) Banquo accompanies Macbeth to victory in the battle against the rebels and traitors;
b) Banquo accompanies Macbeth on their way back to the king when the witches appear on the heath. The witches prophesy about Macbeth's as well as Banquo's future. Banquo notices Macbeth's awkward responses to the witches' proclamations.
c) Banquo accompanies Macbeth to appear before King Duncan who admires both of them, but Macbeth is more profusely rewarded and acknowledged than Banquo;
d) Banquo accompanies Duncan during his visit to Macbeth's castle;
e) After the discovery of Duncan's murder, Banquo mouths pious resolution to 'stand in the hands of God', but does nothing towards the redressal of the foul crime and injustice committed;
f) While Macbeth is haunted by the witches' prophetic soliciting, Banquo is also tempted by the prophecy that his issues would be the kings of Scotland. He doesn't confront the usurper king, but rather compromises;
g) Banquo is killed by Macbeth just before the coronation banquet, but the ghost of Banquo, a hallucinatory product of Macbeth's guilt and fear, appears at the Banquet to force Macbeth reveal his crime, and to lead him to the witches for the final round of doom;
h) Banquo is a foil to Macbeth, a sort of alter image. He is deliberately made an incomplete characterization.
i) Banquo has been very curiously used by Shakespeare as a part of the play's elaborate supernatural machinery. Banquo's ghost proves that dead Banquo is more potent a threat for Macbeth than living Banquo.