While Banquo and Macbeth don't make an appearance until Act 1, Scene 3, the audience is given an idea of what to expect about their relationship beforehand, in Act 1, Scene 2, when the captain is reporting on the state of the battle. He focuses mostly on Macbeth's actions, but when Duncan asks if the captains, Macbeth and Banquo were dismayed by the threat against them, the wounded captain says they "doubly redoubled strokes upon the foe" and ended up winning the battle. Through this report, the audience understands that both Banquo and Macbeth are brave, even when the odds are against them. Neither of them turned and ran; rather, they stayed together, fought, and won. This shows their loyalty to Scotland, and to one another.
When the two men make their first appearance in Act 1, Scene 3, they are returning from the battle and they encounter the Weird Sisters. Immediately, they are both taken aback by the strange creatures in front of them, but upon hearing the prophecies directed at Macbeth, Banquo questions Macbeth's shock, saying "Good sir, why do you start and seem to fear/ Things that do sound so fair?" (54-55). He then goes on to refer to Macbeth as his "noble partner" and asks the Weird Sisters to also speak of his future, saying he is indifferent to their favors and hate. By calling Macbeth "noble," and inquiring about Macbeth's shock, Banquo reveals that he cares for Macbeth as a person, even beyond as a fellow soldier.
After the Weird Sisters disappear, Banquo confides that he cannot believe what they have seen. The two men then restate part of the prophecies, attempting to make sense of them together. Finally, when Ross and Angus enter and proclaim Macbeth the new Thane of Cawdor, Macbeth and Banquo are again taken aback. In a brief aside, Macbeth asks Banquo if he has hopes his children will be kings, now that Macbeth has become the Thane of Cawdor (as the witches suggested he would). Here we see a good sign of their relationship, as Banquo responds:
But 'tis strange
And oftentimes, to win us to our harm,
The instruments of darkness tell us truths,
Win us with honest trifles, to betray's
In deepest consequence. (134-138)
This shows that the two men confide and trust in one another, and it also shows another aspect of their relationship, as Banquo offers wisdom and advice to Macbeth. Finally, at the end of the scene, Macbeth again pulls Banquo aside and says that he would like to again speak privately about the Weird Sisters when they have a chance. Banquo consents, and the men go on their way.
Even before we meet them, Banquo and Macbeth are established as very strong and loyal to Scotland. Upon their entrance, they are shown to be loyal to one another. As Macbeth turns on Scotland, ultimately ruining the country under his reign, Banquo shifts from friend to foil.
Readers are not "introduced" to Banquo until act one, scene three, of William Shakespeare's Macbeth. In this scene, Banquo and Macbeth are greeted by the three witches and offered prophecies.
Macbeth's prophecies state that he will be the Thane of Cawdor and king. Banquo's prophecies state he is lesser than Macbeth, yet greater (lesser in title, better in character), not happy, yet happier (since he is not acknowledge by the king, yet a good man), and his sons will be kings, yet he will not.
After Macbeth hears his prophecies, he seems worried. Banquo, confused about Macbeth's fear, tries to console him. Banquo proves to offer himself to Macbeth as a true friend and follower ("Speak then to me, who neither beg nor fear / Your favors nor your hate"). The conversation between Macbeth and Banquo prove that they support and care for one another.