Saku Tatsuhiko is a minor character introduced in Toaru Majutsu no Index. He is the leader of BLOCK. On October 9 he tried to lead the faction from the Dark Side of Academy City along with 5,000 mercenaries from the outside of the city as part of their plan to attack Aleister Crowley. His plans are later thwarted by the efforts of GROUP.

(Source: Toar Majutsu no INdex Wiki)