Though he was only a young boy in the time of the Eugenics Wars, they had a profound impact on his outlook on life. He felt, for instance, shame toward his country for being so seemingly unaffected by and unconcerned with the plight of the rest of the world. (VOY: "Future's End") He was also secretly jealous of the superior abilities of the Augments.
Brennan was adept at several musical instruments, including guitar, piano, drums and recorder. Brennan -- along with his partner in life, love, and music, Sara -- formed one of the first groups to give a concert at the Millennium Gate Stadium, a multi-sensory performance venue with a self-contained environment, designed to let the audience "feel, smell, and taste the music."
Brennan had a casual friendship with a time-traveling space captain, who'd been "sent from the future to warn him." Brennan was never quite certain, however, about what precisely he was being warned.