I absolutely adore how this kind of question gets the reader/audience to really THINK! In regards to Banquo, four adjectives that would describe him the best are loyal, doubtful, suspicious, and royal. These adjectives sound strange, don't they? Now let me explain them. Banquo is nothing if he is not loyal to the crown of Scotland. Banquo would absolutely NEVER think of harming the king in any way. In this way he is truly loyal to the crown. Also, upon meeting the witches for the first time and hearing them speak, Banquo is doubtful of their words and even that anyone should listen to them! Next, Banquo is most certainly suspicious that Macbeth committed the foul murder of Duncan. This is obvious when Banquo says the following:
Thou hast it now: King, Cawdor, Glamis, all, / As the weird women promis'd, and I fear / Thou play'dst most foully for't.
This suspicion in addition to the witches' prediction that Banquo would be the ancestor of kings cause Macbeth to act most foully yet again and have Banquo killled. (Thank goodness Fleance escapes!) Finally, Banquo is royal, ... or at least his family will be. It is the escape of Fleance that allows Banquo's descendants to be kings!