"Aach" designated a male Human operative who functioned as an agent for the Rebel cause during the Galactic Civil War. He notified Garm Bel Iblis concerning the Death Star plans in 2 BBY. Functioning as a dedicated messenger solely for the most important Alliance affairs, Aach reported directly to Bail Organa.
Although he expressed his desire to join Bel Iblis on his assignment to Darkknell, even at the cost of his return to Alderaan, the senator was convinced that Aach's services would be more valuable elsewhere, and sent him back to Organa.
Timothy Zahn and Michael A. Stackpole featured Aach in the Star Wars Legends short story "Interlude at Darkknell", included in the Tales from the New Republic anthology released in 1999. Following the 2014 reset of the Star Wars timeline, the 2016 canon sourcebook Star Wars: Rogue One: The Ultimate Visual Guide, which accompanied the movie Rogue One: A Star Wars Story, revealed that Cassian Jeron Andor, a central character in the film, had also employed the codename "Aach" while working as a senatorial informant on Darkknell.