A person who is progressive wants to effect change. Progressivism as a social movement dates all the way back to the early 1900s when people began rejecting the idea of Social Darwinism, which taught survival of the fittest, that those who are in positions of wealth and power are so because they genetically deserve to be in such positions. In contrast to Social Darwinists, Progressives argued that society's problems of "poverty, violence, greed, racism, and class warfare" could be fixed through "providing good education, a safe environment, and an efficient workplace" (The George Washington University, "The Progressive Era (1890 - 1920)"). Several characters in Harper Lee's To Kill a Mockingbird can definitely be seen as progressive, two of which are Atticus Finch and Dolphus Raymond.
Atticus Finch's progressivism is best seen in his fight against racism. When Tom Robinson is accused of rape, though Atticus dreads taking on a racial case because its outcome is inevitable due to the town's prejudices, he willingly sets out to defend Robinson. When Scout asks Atticus why he is defending Robinson, Atticus explains he is doing it for the sake of justice, as we see when he says, "..., if I didn't I couldn't hold up my head in town, I couldn't represent this country in the legislature, I couldn't even tell you or Jem not to do something again" (Ch. 9). For this time period, Atticus's perspective that Robinson is innocent and that it would be unjust not to defend him can certainly be considered progressive, and attitudes like Atticus's will eventually effect social change.
A minor progressive character is Dolphus Raymond, a white man ostracized from society for living with an African-American woman he would marry if he were legally permitted. Raymond has allowed Maycomb to believe he is a drunkard so that the racists of the town have an excuse to attribute to his choice to live on the fringe of society. As Raymond explains to Dill and Scout, though pretending he's a drunkard is dishonest, "it's mighty helpful to folks ... they could never, never understand that I live like I do because that's the way I want to live" (Ch. 19). Again, rejecting society's racism in favor of one's own feelings and desires can be considered progressive, and attitudes like Raymond's will eventually effect change.