Bernard Kittridge was a staff sergeant in the U.S. Marine Corps, medically discharged after losing a leg in Afghanistan. After his recovery, he lived in the Denver area.
During the early stages of the outbreak, he became famous as "Last Stand in Denver" after videos he made killing virals, using a sniper rifle from his fortified high rise, are uploaded to the internet. Eventually he is located by someone in the government—presumably military—and they come after him. They cut his power, but he is able to escape.
Kittridge ends up at Mile High Stadium, where the government had set up a refugee center. There he meets April and Tim Donadio, an eighteen-year-old girl and her eight-year-old brother, and Danny, a mentally-challenged school bus driver. He joins them and leads a group of survivors to a refugee center in Iowa. Along the way, he develops a romantic attachment to April. When the center is overrun by virals, Kittridge is killed while trying to save Tim.
Although she doesn't know it, April is pregnant with Kittridge's child—a boy she names Bernard Donadio—when she and the others from their original party escape the refugee center. Though it is never stated, the implication is that they are the original ancestors of Alicia Donadio.