aka: Bathym, Mathim, Marthim
Strongest: Middle of June
Demon Summoning: Daytime, purple candles, lead/uranium, mistletoe
Like many demons, Bathin appears in multiple forms. Typically the first form is intended to intimidate amateur summoners in order to keep availability open for those who really need help and are strongly dedicated to this type of communication.
Bathin's first form fits the bill quite well. A tall, strong man with the tail of a serpent, he rides in on a pale horse, which is well known to be an ancient symbol of death. This is mostly an act for those who are hoping to see a "scary demon", and a bit of an inside joke it seems among the demons themselves. (Fellow demoness Beleth also has a similar initial appearance.)
Bathin's true form is actually much more recognizable. She is the same spirit as the ancient Egyptian goddess Nephthys (aka Nebthet). Those familiar with Egyptian mythology know that Nephthys is the wife of Seth, the god of chaos, destruction, and darkness. Her husband Seth is most likely the same spirit as the demon Sitri, and as such, the two often appear together in summonings.
Though, like her husband, she is intrinsically connected to death and darkness, she is unlike her husband in that she focuses her energy on teaching mankind the ancient secrets of the gods (rather than focusing on death and destruction). In particular, she teaches the secrets of using herbs and stones in the magical arts. She is very tall and strong for a woman and has long, curly blonde hair.
Bathin is undoubtedly an expert in stones and herbs, but her true specialty is teaching the secrets of astral projection, which allows people to transport their own astral forms to different locations in the world. Experts at astral projection can even project themselves anywhere in the universe. This is, in my opinion (along with many others) the most likely way that demons can travel from their world to ours so quickly and easily.