What did the true Sith look like?

All Sith castes were red-skinned humanoids with distinctly sharp, predatory features and tentacle beards. Among the Sith, the stroking of a right cheek tendril was a sign of thoughtfulness. After interbreeding with the Dark Jedi exiles, the original members of the species became known as Red Sith.

Is every Sith a Darth?

A few Sith, most notably, Darth Revan, kept their given names. The title Darth was not used in all eras of the Sith Order. By the time of Darth Bane, around 1,000 BBY, Darth had fallen out of use in order to prevent conflict among the many Sith, and most Sith used the title Dark Lord.