Gilbert Stein is a significant supporting character in The Darwin Incident, primarily defined by his role as the adoptive father of the protagonist, Charlie. His background is that of a professional researcher specializing in chimpanzees, which provides him with a unique and informed perspective on his son's complex nature as a humanzee. He and his wife, Hannah Stein, live in the American provincial town of Shrewsville, where they made the decision to raise Charlie as their own child after he came into the care of the government.

In terms of personality, Gilbert is depicted as a caring and dedicated parent who has taken on the immense responsibility of raising a half-human, half-chimpanzee child. He is committed to providing Charlie with as normal a life as possible. For fifteen years, he and Hannah home-schooled Charlie, sheltering him from the outside world before the decision is made for him to enroll in high school to be among other people. This action reveals his motivation is to help Charlie integrate into human society, believing it is an important step for his development.

Gilbert’s role in the story is that of a protective father and a stabilizing influence in Charlie's life. As his father, he represents the loving and supportive home environment that stands in stark contrast to the dangerous and extremist outside world, particularly the Animal Liberation Alliance, a terrorist group that wants to recruit Charlie. He is part of the small, close-knit family unit that provides Charlie with his sense of belonging and safety. His key relationships are primarily with his wife, Hannah, with whom he co-parents, and with his son, Charlie. He is a central figure in Charlie's personal life, offering guidance and support as the young humanzee navigates the challenges of high school, including bullying and suspicion from others due to his differences in appearance and ability. Regarding his development and any notable abilities, no direct information is available.