Themis is a Titaness, but one of the good ones that ended up siding with the gods in the all out Titan War. Like the rest of the Titans, Themis is a daughter of Gaia and Uranus. Her role is being the Titan of Justice and Order, and she takes that job seriously, making her a rather stoic and solemn Titan. She also has a role as being the Titaness of prophecies, which ties more in to later events in her life that we'll get to in a moment.
Chances are that you've seen depictions of her throughout your life. If you've ever been to a court, which I hope you haven't, or if you've seen movies about trials or lawyers or something judicial, then you've probably seen the lady, blindfolded, who is holding the scales. Yeah, that lad