Nosramus

Nosramus is a white haired alchemist living and studying in the dungeons of Fear and Hunger. He doesn't seem to radiate immediate danger or aggression, but something about his stature seems almost otherworldly or ancient.

Lore
He was the fifth member of the Fellowship of the current New Gods. The now called "Forgotten One" has little written about him and, depending on which version of the story is followed, was either deceived by the older gods or tried to trick the Fellowship.

However, it is revealed by Valteil in Enki's Ending S run, that Nosramus did not ascend like the others in the Fellowship, but took the path of enlightenment instead, explaining why he didn't have such a dour mood compared to Valteil. He would eventually find the secrets of immortality, hence why he has a prolonged life.

He would eventually give Enki a Spirit Anchor, enabling the man to see his reflection in the Void but still ground himself to reality and not give in to the lust of power that godhood provided. And so Enki followed Nosramus' path: the path of enlightenment.

Behaviour
Depending on your actions, Nosramus will be either friendly or hostile towards you (not in the sense of attacking you, but more so ignoring what you have to say). For him to be friendly towards you, introduce yourself when you first meet him in the cave west of the Mines, and don't hide in the shadows, or else that will offend him and potentially end any sort of interaction with him until you start up a new playthrough. It is especially recommended to not do this if you are trying to do Enki's Ending S run as interacting with Nosramus is needed to acquire the one needed object to achieve the ending.

Once you are within his room after defeating his bodyguard and faithful undead servant, simply go to him and talk. Don't even bother to try looking into his chest. He will warn you initially to back off from it but if you just go ahead and open and take whatever is in it, he will no longer talk to you and end any potential dialogue and lore information you might get from him and end the hardmode Enki Ending S run. After moving to his 2nd hideout nearby Salmonsnake's pool, you can check this chest freely, he will not be bothered.

Trivia

 * His name might have been taken or inspired by a man named Michel de Nostredame. His name is usually Latinized as Nostradamus. The man was a French astrologer, physician, and a reputed seer. He is known for his book Les Prophéties, which is a collection of 942 poetic quatrains that allegedly predicted future events and prophecies that will happen after his time.