Luo Ji is an astronomer and sociologist who enjoys a carefree life until he is mysteriously thrust into the center of Earth's defense strategy. He is named one of the four Wallfacers, individuals granted absolute authority to devise a secret plan against the alien invasion. Instead of working, he initially demands a luxurious estate and tasks his bodyguard with finding his ideal romantic partner.

Shi Qiang is a gruff but highly capable detective assigned to protect Luo Ji. He is practically minded and serves as a bodyguard and grounding force for the reluctant Wallfacer. He ignores bureaucratic rules to ensure Luo Ji's safety and procures whatever the astronomer requests without judgment.

Zhang Beihai is a naval officer reassigned to the newly formed UN Space Force. He exhibits a strict "triumphalist" attitude regarding the coming alien conflict, publicly demanding that the military maintain absolute confidence in human victory. He evaluates his colleagues based on their ideological purity and dedication to the war effort.

Ye Wenjie is an astrophysicist and central figure in the Earth-Trisolaris Organization (ETO). In a flashback at a cemetery, she suggests that Luo Ji study "cosmic sociology," a theoretical approach to how civilizations interact across the universe, setting the entire plot into motion.

Frederick Tyler is a former US secretary of defense with a background in international strategy. As a Wallfacer, he is tasked with designing a covert military defense against the Trisolarans, focusing on swarm-style fighter fleets and seeking unusual alliances to build his forces.

Manuel Rey Diaz is the former president of Venezuela who successfully resisted foreign military intervention using guerrilla warfare. He is appointed as a Wallfacer to develop a secret plan, requesting massive nuclear bombs for his aggressive defense initiative.

Bill Hines is an English neuroscientist and former EU president. He focuses his Wallfacer resources on mapping the human brain, hoping to accelerate human intellectual evolution to outmatch the alien surveillance devices known as sophons.

Keiko Yamasuki is a brilliant neuroscientist who partners with her husband, Bill Hines, to map the mind. She aids his Wallfacer initiative by studying critical thinking and observing the psychological effects of the impending crisis on human subjects.

Zhuang Yan is a recent university graduate and artist who perfectly matches Luo Ji's detailed description of his ideal girl. She is brought into the Wallfacer project to act as Luo Ji's companion, eventually marrying him and moving into his secluded estate.

Wu Yue is a naval officer who commands a battleship before being moved to the Space Force. He holds a pessimistic, "defeatist" view of the impending Trisolaran conflict, which strains his working relationship with his triumphalist colleague and affects his military career.

Zhang Yuanchao is a retired chemical plant worker living in a Beijing apartment building. He and his neighbors spend their days watching the news, arguing over the ethics of the controversial "Escapism" movement, and worrying about resource shortages.

Yang Jinwen is a retired military teacher residing in the same Beijing apartment building as Zhang Yuanchao. He debates the Trisolaran crisis with his neighbors and makes plans to have his remains shot into space to avoid the alien invasion.

Miao Fuquan is a wealthy former mine boss from Shanxi who lives in the same apartment building as Zhang and Yang. He invests in an illegal "Escape Fund" designed to ensure his descendants can flee Earth before the invasion arrives.

Shi Xiaoming is the son of Detective Shi Qiang. He becomes involved in the illegal Escapism movement by operating a controversial Escape Fund that targets citizens desperate to secure a future off-planet.

Chang Weisi is a military leader who oversees the formation of the Chinese Space Force. He commands officers like Zhang Beihai and coordinates Earth's early space defense protocols.

Ding Yi is a physicist who understands how the alien sophons inhibit Earth's research. He advocates for alternative scientific methods and later travels into deep space to examine incoming alien technology.

Dongfang Yanxu is a captain within the Asian Space Fleet. She commands the starship Natural Selection and interacts with military reviewers assigned to ensure ideological discipline among her crew.

Mr. Kent is a UN official assigned to act as Luo Ji's handler. He manages the Wallfacer's unusual requests and serves as a liaison between Luo Ji and the Planetary Defense Council.

General Fitzroy is a military leader working with astronomers to track the incoming Trisolar fleet. He oversees the commandeering of the Hubble II space telescope for defense surveillance over the protests of the scientific community.

Albert Ringier is an astronomer who works with General Fitzroy on the Hubble II project. He maps distant stars for Luo Ji's secretive Wallfacer experiment.

Xia Xia is the young daughter of Luo Ji and Zhuang Yan. She lives with her parents in a secluded luxury estate and serves as a primary motivation for her father's work.

Ben Jonathan is an administrative official who serves as a Wallfacer handler, managing logistics and political structures for Luo Ji during Earth's ongoing strategic defense.

Lan Xi is the chief psychologist aboard the starship Natural Selection, responsible for treating extreme homesickness and paranoia among the crew during deep-space missions.

Mike Evans is one of the deceased leaders of the Earth-Trisolaris Organization (ETO). His past communications with the aliens accidentally revealed that human thoughts are secret, causing the Trisolarans to fear humanity's capacity for deception.

Bai Rong is a young adult author and former romantic partner of Luo Ji. During a road trip, she encouraged him to visualize his "fantasy girl," inadvertently sparking his lifelong obsession.