Quixote is an idealist. Perhaps, he ends up learning the harsh truth of the world, but his defining elements are ones of idealism. He pursues a conception of the good that only exists in his own mind and he has little care for how others perceive him. Quixote lives in the world of dreams. His defense of Dulcinea's honor might be completely antihetical to what reality presents, yet that is irrelevant to him. What Quixote believes is in his own mind and his own pursuit of it is idealistic, in its very nature. The idealism present in Quixote is what makes the novel so compelling in that he passionately speaks to what it means to be human in terms of believing in one's dreams and appropriating the world in accordance to one's own subjectivity. In this element, one sees the overwhelming idealism present in the character of Quixote.
Don Quixote is most certainly an idealist. To quote the words of the musical, he "dreams the impossible dream." While many of his desires are realistic, his choice to pursue them and certainty in attaining them is idealistic.
Some might consider Don Quixote insane, as he really does go overboard with some of his actions. However, he can also be seen as simply being self-indulgent. He has dreams he pursues, which is more than most of us can say. He will pursue them at any cost.
One of the most important themes of the novel is love, which is important because often people become overly idealistic when they follow love. There is an old saying that we see those we love with rose colored glasses, and one could also say that Don Quixote sees the entire world with rose colored glasses! Hence, he is the perfect idealist.