They are said to take the shape of giant snakes, sometimes with a human head. Other times they appear as misty ghost-like humanoids. They may keep their snake pupils and forked tongues and share some things in common with their original reptilian form, like the need to use external warmth and the habit to hibernate in winter/cold temperatures.
They are secretive and mysterious, when they appear at all. They seem quiet and reserved, perhaps even shy.
Not much is known about the elusive Hebi. It seems to be one of the rarest, most mysterious types of non-ghost Ayakashi in the world.
*the actual reason why the Hebi aren't featured much in Ink Stained (apparently, people think they're cool) is because Break is ophidiophobic. She cannot even look at pictures of snakes without getting hives all over.