A foil in literature shares some similarities with the protagonist, but possesses characteristics that emphasize the opposite characteristics in the protagonist. Although disagreement exists among some literary scholars, the most common interpretation of Banquo's character is that he does serve as a foil to Macbeth in the play. In regard to your thessis, you might consider opening it up a bit to be less specific and more inclusive. Some suggestions:

• Banquo's identity and personal character make him an effective foil to Macbeth.
• Because of Banquo's elevated social status and personal goodness, he serves as an effective foil to Macbeth.

With either of these thesis statements, you can develop an essay that establishes and explains these points in support:

• As the play begins, Banquo and Macbeth share equal power and position; they are both Scottish generals, valiant in battle and united in supporting King Duncan. (These are the similarities between them that later bring Banquo's character into contrast with Macbeth's.)
• Banquo's personal character is one of goodness that serves to emphasize Macbeth's evil. He lacks selfish personal ambition, turns away from evil, and remains loyal to King Duncan.

In terms of Banquo's basic goodness, it might be interesting to note his relationship with his son. He is a loving, protective father to Fleance. This trait in his character perhaps emphasizes Macbeth's coldness and cruelty in ordering the slaughter of children, all of Macduff's "pretty ones."

The links below will take you to some helpful information. Good luck with your study!