Dunois is Joan's best friend. When the wind changes upon her arrival at Orleans, he dedicates himself to her, saying, "The wind has changed. […] God has spoken. […] I am your soldier" (3.72). Of course, they get along even before this "miracle" happens. From the moment they meet on the banks of the Loire, he's impressed with Joan's courage. They're both soldiers at heart. Dunois identifies with her love of war, though he's not quite as gung-ho as she is. He says, "I, God forgive me, am a little in love with war myself, the ugly devil! I am like a man with two wives" (3.48). Later on he advises her, "You must learn to be abstemious in war, just as you are in your food and drink, my little saint" (5.9). Dunois is constantly telling her things like this. Even if the truth is painful, he lays it out there for her like a good friend should.
Throughout the play, Dunois functions as Joan's voice of reason. A good example of this is after Charles's coronation. Joan can't understand why their powerful allies don't seem to truly appreciate everything she's done for them. "Sim-ple-ton!" says Dunois, "Do you expect stupid people to love you for shewing them up?" (5.13). It seems like he's telling her this out of concern. He knows that Joan is making more enemies than she is aware of and he wants her to be careful.
Another example of Dunois' tough love is when he tells her that he won't help her liberate Paris. They don't have enough soldiers; it doesn't make sense. He tries his best to make her see reason saying, "God is on the side of the big battalions" (5.82). Joan does it anyway, getting caught in the bargain, because she places more value in the advice of her voices than her friend.
The toughest love of all comes when Dunois says that he won't rescue her if she gets caught. He comes back in the dream sequence epilogue and defends his choice to abandon her, saying that there was simply nothing he could do. She was in the Church's hands. There was no way he could intervene without getting burnt himself. Ultimately, his practical nature overruled any kind of swashbuckling rescue attempt.
Even though he defends his choice to abandon her, it still seems like he feels bad about it. He tells her that he wrote a letter in her defense to help clear her name. Also, when he disappears for the last time, he says, "Forgive us, Joan: we are not yet good enough for you" (E.158). In the stage directions, Shaw describes Dunois as "a good-natured and capable man who has no affectations and no foolish illusions" (3.1). Seems like Shaw knows his character pretty darn well.