When we first meet Charles, he's a total wuss. His advisors bully him and basically do whatever they want. He complains, "You all think you can treat me as you please because I owe you money, and because I am no good at fighting" (2.31). They put up with him because of his royal blood, and even that is in question since his mom disinherited him. You'll find a discussion of Charles's political plight under "Setting." Basically, he's in serious trouble – surrounded by enemies and afraid to do anything about it. He has a legitimate claim to the throne, despite his mother's betrayal, but due mostly to his own spinelessness, he is still the Dauphin and not King Charles VII.
When Joan shows up, Charles informs her, "I am quiet and sensible; and I don't want to kill people: I only want to be left alone to enjoy myself in my own way" (2.154). Joan pretty much tells him that he's going to have to suck it up and make some battles happen. It's either be a king or be a bum. As you might imagine, he decides on being a king.
And why not, right? All he's got to do is sit back and chill in his castle, while she goes off and does the hard stuff. After her little pep talk, he does manage, with her support, to summon enough courage to dismiss La Trémouille as head of the army and give command over to Joan. Of course, later on, after she's gone through all the trouble of getting him crowned at Rheims, all fighting spirit seems to have left him. He's right back to wanting to negotiate again. All of Joan's victories in his name really haven't changed him very much.
In the epilogue, however, we see a different Charles. Joan comes back to him in a dream twenty five years after her execution. King Charles tells her, "Do you know I actually lead my army out and win battles? Down into the moat up to my waist in mud and blood. Up the ladders with the stones and hot pitch raining down. Like you" (E.40). Everybody calls him Charles the Victorious now. (Hey, good for you, Charlie.)
Still, though, it's not like he's become great and noble or anything. He did have Joan's name cleared, but only because he didn't want people going around saying he was crowned by a witch. In the end, Charles offers the best defense of his own character:
"I have done less harm than any of you. You people with your heads in the sky spend all your time trying to turn the world upside down; but I take the world as it is, and say that top-side-up is right-side-up; and I keep my nose pretty close to the ground." (E.65)
It seems that Charles sees himself as a pragmatist. He's just your average ordinary king, trying to get by in this crazy world. He adds, "what king of France has done better, or been a better fellow in his little way?" (E.65). Fair enough, Charlie – fair enough.