Hermes Trismegistus, or just Trismegistus, is quoted as the founder and pioneer of alchemy, astrology and magic. He is a mysterious entity, either known as a god or a human magician. He has been seen as a teacher of Moses, the inventor of alchemy, and the founder of occult schools throughout history. The origin of the name Trismegistus, meaning "thrice-greatest," refers to the three subjects that he was a pioneer of.
As a divine source of wisdom, Hermes Trismegistus was credited with tens of thousands of highly esteemed writings, which were reputed to be of immense antiquity. Clement of Alexandria was under the impression that the Egyptians had forty-two sacred writings by Hermes, writings that detailed the training of Egyptian priests. The neoplatonic writers took up Clement's "forty-two essential texts". The Hermetica is a category of papyri containing spells and initiatory induction procedures Modern occultists claim that some Hermetic texts may be of Pharaonic origin, and that the legendary "forty-two essential texts" that contain the core Hermetic religious beliefs and philosophy of life, remain hidden in a secret library.
Hermes Trismegistus is said to be the combined form of the Greek god Hermes with that of the Egyptian god Thoth. Greeks in the Ptolemaic Kingdom of Egypt recognized the equivalence of Hermes and Thoth through the interpretatio graeca. Consequently, the two gods were worshiped as one, in what had been the Temple of Thoth in Khemenu, which was known in the Hellenistic period as Hermopolis. The Egyptian priest and polymath Imhotep had been deified long after his death and therefore assimilated to Thoth in the classical and Hellenistic periods. A Mycenaean Greek reference to a deity or semi-deity called Tris Hḗrōs, ("thrice or triple hero") was found on two Linear B clay tablets at Pylos and could be connected to the later epithet "thrice great", Trismegistos, applied to Hermes/Thoth.
Cicero enumerates several deities referred to as "Hermes": a "fourth Mercury (Hermes) was the son of the Nile, whose name may not be spoken by the Egyptians"; and the fifth, who is worshiped by the people of Pheneus, is said to have killed Argus Panoptes, and for this reason to have fled to Egypt, and to have given the Egyptians their laws and alphabet: he it is whom the Egyptians call Theyt. Sayyid Ahmed Amiruddin has pointed out that early Christian and Islamic traditions call Hermes Trismegistus the builder of the Pyramids of Giza and has a major place in Islamic tradition.
Due to his connection with alchemy, he is an equivalent to Mercury, just like Hermes.
Junpei Iori's Persona Hermes evolves when Chidori uses her ability, Spring of Life, to resurrect Junpei and confesses her love for him. Chidori's Persona, Medea, then fuses with Hermes to give birth to Trismegistus. (Shown in Persona 3: Portable)
Trismegistus' helmet and armor is a reference to the Ripley Scroll, a hermetic treatise purporting to tell of the process to fabricate the Philosopher's Stone, the powerful elixir said to bestow eternal life and transmute lead into gold. The references are the birdlike helm and wings, in the red stone it holds in its beak, and its name itself, Trismegistus being the honorary title of the famed alchemist Hermes; also, the Ripley Scroll makes continuous reference to the "Bird of Hermes."
In Persona 3 Portable, there is a typo in Trismegistus' awakening, there is an "o" instead of a "u," so instead of being "They gave rise to a new power, Trismegistus!", it's "They gave rise to a new power, Trismegistos!"