Allan Brown

Allan Brown is an evolutionist geneticist who has spent most of his life studying the process of evolution. He is a lot socially awkward and only talks about his work, to which he is addicted to. However, he works well with the other scientists to determine weaknesses of the aliens. Allan has managed to discover some things the aliens developed and from that he deduced some conditions that apply on their planet (the planet the leeches drew to them to occupy and later destroy). Having discovered these conditions, he was able to use certain weather conditions against them. He had no family prior to the apocalypse
Given the first chance he has, he wants to examine a leecher. He does so and realizes the leecher has no pulse. After closer examination, he finds the unusual teeth are part of a larger organism that spreads throughout the entire head and reaches the teeth. This organism is the only living thing on the body. He then cuts the head of the infected and it continues to be alive. Everyone watches in shock, as Allan turns around and throws up
He also examines some aliens when they first encounter them, and says they have developed an exoskeleton which seems like they are from earthh. he says this is just a theory, but these creatures should be totally different than what we see on earth. Someone asks then what he means, and allan says that there is a big chance that the "aliens" originate from earth. Someone thinks about the russians being behind this, but Norman intervenes and says these are all just theories. For all they know, maybe species are supposed to evolve in these ways and by some miraculous chance they look like earthly creatures. Maybe they do not consist of DNA even. Or even if they do, another theory suggests that the first life on this planet came with meteorites. The fact that this life may also consist from DNA may prove this theory to be right, and life on earth and the aliens may share long past ancestors. Allan says this makes sense too, but the similarities with animals of the earth are too many to not see.
When at first he  develops depression, he starts by not speaking much and just standing there. When asked if he haas something, he always says no. He is never focused on his work. He doesn't eat much. Sometimes he wakes up at night and in the days he is almost always sleepy, and this affects his work too. When he and Norman will dissect a leecher, he will be a little off. Just before he tries to commit suicide, he even asks if he is a burden since he can't kill leechers, can't go find food and only eats it, and can't reproduce anymore. William reassures him that's not the case, and says they won't leave anyone behind depending on their usefulness. Allan is not convinced yet, and attempts to commit suicide. William saves him and notes his importance to the future of the hman race, and Allan slowly heals after this
Later he loses the will to live, because he lost his family thirty four years go when his wife took their daughter annd left, and he now doesn't want to live to see the world end. He tries to commit suicide by letting himself fall behind, but William stops and saves him despite his wish. Then, William announces that Allan is one of the brightest scientists left in the world, so it is not his choice to die because it affects anyone and the future of humanity
Allan dies later in an event not yet decided. His death affects everyone in the group as he had grown to be likeable by everyone.