My final headshot from Tiana for a while, I've sold my soul to them for long enough (for now) ;>>

Sigma is an odd one. They're an alien called a Gray, psychic, genderless creatures who communicate exclusively via telepathy, or a translator implanted into the skull.
Due to their general lack of a face aside from eyes, Sigma is quite difficult to read, and can often be caught staring at nothing in particular for an uncomfortably long time.
They wear a surgical mask to further obscure their features, perhaps just as a fashion statement. Or maybe they're hiding something?

Tiana's FA: https://www.furaffinity.net/user/tianaandotherocs/