Trismegistus

H. T. Trismegistus (H・T・トリスメギストス) is a character 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.

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.

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.