Which non-human character does the most to shine a light on humanity?

Science fiction and fantasy are full of aliens, demons, robots and other non-human creatures... who show us what it really means to be human. But which non-human character is the best at illuminating the nature of humanity?

Please include a picture or clip (or book cover) featuring this character. Also, the name of the series, and why this person reveals so much about humanity, would be awesome. Thanks!

Top image: Buffy the Vampire Slayer