D'Artagnan undergoes the greatest emotional journey in the novel. While Porthos remains always affably reliable, Athos stays always sensitive and caring, and Aramis continually conniving, D'Artagnan actually learns to swallow his pride by the end of the novel. (Chapter Fifty-Three to be exact.) Of course, nobody ever said anything about this being a positive transformation – check out his speech to King Louis in which he says, "In taming me you have lessened me" (53.64).
In case that doesn't convince you, think about the fact that when D'Artagnan dies, the novel ends. This might be symbolizing that D'Artagnan is the reason behind the narrative. Once he is gone, there is no need to continue the series.