angel's advocate
One who looks for and argues in support of the positive aspects and benefits of a certain argument, whether or not he or she believes them to be true. it is the opposite of a "devil's advocate," who argues against something for the sake of argument, not due to a personal opinion. I know a lot of people oppose the building of a new railway, but let me play angel's advocate for a second and tell you about all the ways it will improve our city!