I think focusing on the 'key' survivors might miss the point a little. The book's a series of interconnected reports, and the survivors are just the people giving those reports. Calling them 'characters' in a traditional sense feels off to me. For instance, the doctor in China, Dr. Kwang Jingshu, who first encounters the virus, is a key witness, not a protagonist you follow. His account is about societal breakdown and government cover-up, not his personal journey.
Then there's the Russian soldier, Jurgen Warmbrunn, whose story is about the military's failed response. You learn about the disaster through his strategic hindsight. The Japanese otaku-turned-samurai, Kondo Tatsumi, offers a cultural and personal transformation angle, but again, you only get his single interview.
For me, the most compelling ones were the more ordinary voices, like the downed pilot in the Louisiana swamp or the woman trapped in her apartment. Their survival felt more visceral because it wasn't about being a 'key' player in the grand scheme, just about enduring a day at a time. The book's power is in the mosaic, not in individual hero arcs.
I actually found myself forgetting names but remembering situations, which I think is what Brooks intended.