Trismegistus

Trismegistus (トリスメギストス), usually H. T. Trismegistus (H・T・トリスメギストス) (short for Hermes Thoth Trismegistus (ヘルメス・トート・トリスメギストス)),   is a Transcendent of the Bridge Builders Cabal, introduced in Souyaku Toaru Majutsu no Index.

Etymology
Trismegistus, specifically ("Thrice-Great") - syncretized from the Greek  and the Egyptian, is a legendary figure whose attributed works form the basis of Hermeticism.

The initials before Trismegistus are derived from the first letters of either the syncretized deities behind Hermes Trismegistus (the default above) or the deities he is currently invoking (e.g. Z. I. Trismegistus with Zeus and Indra).

Appearance
Trismegistus is a black-haired man, described as looking like a gloomy young butler. He wears an old-fashioned black coat with a brown lining, a light blue tailcoat,   a white inner shirt, a brown cravat, dark grey trousers, white gloves,  black shoes and triangular brimmed hat. He also has a black cane, less than a 1m long, hiding a sword.

Personality
Trismegistus is calm and polite on the surface, acting as a butler to Alice Anotherbible, calling her his 'beloved' master (afraid of her anger as a tyrant in his heart) and himself merely a servant. However, he is also ruthless against any obstacles or threats, and if he determines something to be a dangerous element, he will immediately move to eliminate it without holding back.

Trismegistus bases his actions on what he believes to be "common sense" (frequently incorporating the phrase or variants in his speech). He despises things which would distort "common sense", driven by emotions, superstitions and other factors,  and has expressed dismay when his sense of "common sense" is shaken. He believes that the Transcendents should deal salvation evenhandedly.

Background
The legendary figure Hermes Trismegistus ("Hermes Thrice-Great"), a syncretization of the Greek god Hermes and the Egyptian god Thoth, was the attributed author of works such as the Hermetica, which form the basis of Hermeticism. Outside the story and legend behind it, the name Trismegistus was something like a shared penname which ancient scholars and academics used to release their papers anonymously.

Besides the influence of Hermeticism (and thus Trismegistus) on the Golden Dawn (a.k.a. the Hermetic Order of the Golden Dawn), aspects tied to Trismegistus are also present in Thelema and Magick, with Hermes and Thoth are equated in Aleister Crowley's occult dictionary Liber 777,  as well as Thoth's name being used for the Thoth Tarot and the Egyptian god of writing also being associated with the Holy Guardian Angel Aiwass.

As for the Transcendent going by the name Trismegistus, his exact background before and after joining the Bridge Builders Cabal is currently unknown, though his fixation on common sense, his mentions of the and how superstitions creep into academia and distort common sense, and remarks on how no matter how logically one explains their academia theories there was nothing one could do when people brush them aside based on emotion, seem to show Touma glimpses as to what led him down his path.

Post-Handcuffs Arc
When Aradia was guiding Anna around their base, she prioritized introduction to Alice, saying that Trismegistus could wait. After the events surrounding the crash of the Overhunting, as well as the battle in the Vanishing Tunnel, Alice was brought back from Academy City by Anna. Greeting Alice upon her return, H. T. Trismegistus and the others in the group were shocked and dismayed to find that Alice's newfound interest in Kamijou Touma had swayed her from using her magic to benefit their plans.

As the cabal became split over how to tackle the matter of Touma's effect on Alice, between those who wished to kill him or preserve him, Trismegistus sided with the Killers.

New Year's Arc
On January 1st, Trismegistus and Good Old Mary accompanied Alice when she went to greet Kamijou Touma as he and others were making their New Year's Day shrine visits in School District 12. During the encounter, he dissuaded Shokuhou Misaki from peering inside Alice's psychedelic mind.

On January 2nd, Trismegistus greeted Alice when she brought Touma's group to the cabal's newly established consulate, cleaning mud from her clothes and beginning to prepare some games to amuse her for when she showed the guests around her new secret base. After briefly facing Alice's wrath when Touma inadvertently told her about the Killers and Rescuers, Trismegistus explained how the cabal were intending to execute Anna Sprengel for her betrayal, with Mut Thebes present. He was also present for dinner and washed up afterwards.

On January 3rd, Trismegistus encountered Touma and Aradia just after Touma had decided to try and convince the cabal to not to straight up execute Anna Sprengel, with Aradia having just been convinced by Othinus that Touma met her protection conditions. On hearing what the two had to say on the matter, Trismegistus decided that Touma was a dangerous element and immediately moved to kill him at full power, after having struck down Aradia.

After releasing his seal and renewing himself from H. T. to Z. I. Trismegistus, he pursued Touma through the grounds, intending to finish him off before Alice could learn of what he was doing. Trismegistus eventually decapitated him, but Touma had brought enough time for Mary to arrive and revive him, meeting her conditions. The two Transcendents then fought, with Mary eventually releasing her own seal, until Alice arrived to punish them, with only Touma's intervention preventing her from killing them.

Abilities
Trismegistus's exact capabilities are currently unknown, but as a Transcendent of the Bridge Builders Cabal, he is said to be powerful enough to rival the regular magic side on his own. Like some other Transcendents, he uses 'common tone creation' (共通トーン作成) to translate and synthesize his speech into unfamiliar languages.

Trismegistus is capable of producing various items from thin air in a manner resembling that of a stage magician, as well as making them disappear again. Whether this involves storage compartments hidden within his cloak is unknown. Items he has produced include cleaning tools and a long, narrow adult-sized coffin with slits for stabbing swords into.

He uses a cane sword (仕込み杖) in battle,  a spiritual item concealed within his cane, rapidly unsheathing it from its scabbard and slashing, in the manner of. The blade's fast projectile slash can take varying paths and its sharpness can be infused into various phenomena.

Transcendence
Like many other Transcendents in the Bridge Builders Cabal,   Trismegistus can release a seal in order to draw on power transcending that of regular humans, with an incantation involving the declaration of "Risk 4" (リスク4) and "departing human territory" (人域離脱). In the case of Trismegistus, this power is in the form of a 'trio of great beings' (三倍も偉大なる者) and the nature of his seal is a numberless seal (数なき封印).

By invoking the names and powers of two gods which correspond in Liber 777 into his right and left hands, Trismegistus forms a from his hands (the invoked gods) and his main body (himself), implanting the world with a false definition that he is a god too, making his own power into that of a new, false god. When doing this, his initials change to reflect the deities he is invoking (e.g. (H),  (T),  (Z),  (I),  (O),  (M) etc), with the characteristics of his magic changing depending on the combinations of these gods.

Trivia

 * Items which Trismegistus has produced from his pockets or out of mid-air besides the coffin, include a leather notebook, a heavy load of movie pamphlets and TRPG rulebooks, a handkerchief, detergent, a dryer, an iron, and a leather jacket.