Marcus Yallow is a fictional character from the novel series "Little Brother" by Cory Doctorow. He is a tech-savvy teenager living in San Francisco who becomes an activist after a terrorist attack leads to increased government surveillance and control. Marcus is known for his intelligence, resourcefulness, and strong sense of justice. Throughout the series, Marcus uses his knowledge of technology to fight against oppressive systems and advocate for civil liberties. His experiences highlight themes of privacy, freedom, and the impact of technology on society, making him a relatable figure for young readers interested in these issues.