Sergeant Brennan was a Tanith NCO of little narrative importance first encountered during Ghostmaker's the Hollows of Hell when he, along with Obel, are badly wounded following their landing craft's crash-landing. At first Major Rawne is adament that the injured men will slow them down and the best course of action would be to put them out of their misery but Gaunt refuses and due to the efforts of both Meryn and Bragg the two are ferried to safety and evacuated off of Caligula.
Following this Brennan is rarely seen and mentioned only as an ancillary character with little to no effect on any plotline. He is, however, promoted to Sergeant following the Tanith 1st's merging with the Belladon 81st.
Brennan would go on to lead one of the clearance teams during the boarding action of Salvation's Reach where, sadly, he would lose his life alongside Raess, Preed, Mktas and several others when they failed to effectively disarm a Sons of Sek boobytrap.