Very interesting question! In this play one of the main purposes of Banquo's character is to act as a contrast to the character of Macbeth and thus when examining Banquo's character we really need to think about how he is portrayed compared to Macbeth.
A key scene to examine is how both characters respond to the witches' prophecy. The different responses lie in the fact that it is Macbeth that chooses to act on their prophecies, giving in to his overriding ambition, whilst Banquo, although he clearly has ambitious thoughts, does not give into ambition in the same way.
Interestingly, the first time the witches appear to Macbeth and Banquo, Macbeth is startled by their predictions, but it is Banquo that says to Macbeth: "Why do you start, and seem to fear / Things that do sound so fair?" This shows that Banquo is definitely not averse at first to the witches' prophecies. However, this quickly gives way to doubt and scepticism, and later Banquo has troubled dreams regarding the witches and their prophecy, and says to Macbeth: "oftentimes, to win us to our harm, / The instruments of darkness tell us truths, / Win us with honest trifles to betray [us]." This is in sharp contrast with the increasing fascination and obsession that Macbeth (and his wife) have with the predictions of the weird sisters.
Note too how Banquo is not able to sleep (like Macbeth) before the murder of Duncan. We are told that Banquo has had his sleep troubled with dreams of the witches or the "weird sisters". It is clear that these dreams have not helped him to have a good night's sleep - he says:
A heavy summons lies like lead on me,
And yet I would not sleep. Merciful powers,
Restrain in me the cursed thoughts that nature
Gives way to in repose!
We can see therefore that in some ways in this play Banquo acts as a comparison to Macbeth - and a rebuke to Macbeth and the path he has chosen to take.
Interestingly, it is the ghost of Banquo that haunts Macbeth (rather than the ghost of Duncan), and one of the ghost's rebukes of Macbeth is how Macbeth responded to the witches' prophecies in comparison with Banquo. Thus the character of Banquo shows us that it is one thing to have ambitious thoughts (and don't we all), but it is something completely different to become a slave to ambition and be led into paths which involve committing murder and treason to satisfy the thirst of ambition.