Banquo and Macbeth are really good friends. It is not a coincidence that when we first time see Macbeth in the play he is accompanied by his friend Banquo. "Our captains, Macbeth and Banquo" that how the king calls them. Furthermore in the novel we always see them together all the time: on the battle field and in the Macbeth's castle. In the beginning of the second act when Macbeth enters with his servant, and Banquo asks "Who's there", Macbeth's answer: "A friend" shows one more time how strong the relationships between two men are.
In my opinion the main characteristic that a real friend should poses is honesty. It is undisputable that friends and family members (more or less) are the people with whom we usually share most of our important thoughts and secrets. Obviously, you want to trust this people and be sure that they will not betray you. In case of Macbeth having a friend who will not betray you is even more important because on the battlefield betrayal might leads to death. The other reason why friends should be honest is that sometimes friends are the only people who can tell you about your strengths and weaknesses. This kind of evaluation is really important because you can then you can work on yourself. .
The other really important quality for a friend is being able to understand and forgive. Sometimes we do or say things that might hurt others. Only the real friend can understand the reason why did you do this. Being able to forgive probably is the greatest gift, not a lot of people have such ability. But you may ask: "Why should we forgive?" The answer for this question is quite simple. You always should try to forgive the others because you are not perfect yourself. Similar to all other people we make mistakes. Sometimes these mistakes are small and insignificant but sometimes they are really big and maybe the rest of your life depends on it. This is an exact moment when you need you friend right beside you.