Mathers, Samuel Liddell MacGregor

Samuel Liddell MacGregor Mathers, primarily referred to as Mathers, is a posthumous character introduced in Shinyaku Toaru Majutsu no Index.

He was a Magician and one of the three founders of the Golden Dawn, and was one of the mentors of Aleister Crowley, whom he personally invited into the cabal. After Aleister sought to destroy all magic, he would become Aleister's enemy and was later struck down by him following the Battle of Blythe Road. Though defeated, Mathers' actions would have a major influence on the events of the Toaru series.

(Source: Toaru Majutsu no Index Wiki)