The Humans:

William Boone: Boone was an army commander in the SI War (the Sino-Indonesian War). After the war he worked as a police captain in Ohio City. He was married to Kate, and they were planning to start a family.

Boone was assigned to oversee the police protection detail for Da'an, the American Companion, at a public event. An apparent assassination attempt was made on Da'an. But thanks to Boone spotting the laser sight and giving a warning, the sniper supposedly killed Jonathan Doors instead. Boone located the sniper, but allowed him to go free when he turned out to be an old friend.

Nonetheless, Da'an was impressed with Boone and asked that he "become a voice in the shaping of Humanity's future" by becoming his inter-species liason. Boone refused at first because of his and Kate's plans for their future, and he was reluctant to uproot to live in Washington. When Boone told his wife about Da'an's offer, she was angry that he had not consulted her first, as she was an admirer of the Companions. But Boone admitted that he was unable to trust the Taelons as they did not make their motivations clear. That evening Kate was killed in her car, in an explosion.

Boone was then approached by Mayor Ruiz, on behalf of the US President. Da'an had made a request that President Thompson persuade Boone to join the Companions security team. Boone listened to Da'an again, and this time, he accepted the position, as it was something his late wife would have wanted him to do. He bluntly asked Da'an what the Taelons intentions are towards Earth. Da'an replied that Humanity was at an "evolutionary crossroads" and that the Taelons hoped to gain a better understanding of their own evolution by studying the people of Earth. When he accepted, ex-FBI Agent Sandoval told Boone that he would be expected to have a CVI, or Companion Viral Implant, placed in his brain. Boone was tempted by Sandoval's assertion that he would be able to see his wife through the CVI's effect on the memory. He would be able to "live" inside his memories. He was also given a semi-sentient symbiotic weapon created from a race of beings known as the Skrill. Boone's Skrill was named Condor.

After the meeting, Lili Marquette took him to the liberation movement with a view to getting him to work for them. Jonathan Doors, still miraculously alive, told him that Kate was murdered by Da'an to "encourage" Boone to work for him. The Taelon-Human scientist, Dr. Bellman, who worked for Da'an, was also a member of the resistance. She assured Boone that she would create a modified CVI, which would enable him to have complete free will, as the imperative to obey the Companions would be removed. Boone agreed and along with Lili Marquette, he became a double agent.

William Boone spent just one season as Da'an's implant. He helped track down Ha'gel, a Kimera and enemy of the Taelon. Boone was badly wounded by Ha'gel and he was taken to the Taelon mothership to be healed. But while in a healing tank, Zo'or seemingly destroyed him, disintergrating his body, while Sandoval watched.

Boone was the "Everyman", the stand-in for the show's audience. He came into the world of the Taelons and their Human followers from the outside and as he learned, so did we. Although he was on the side of the Liberation, he was not fanatic. He looked on Da'an as a friend, albeit one who could not be completely trusted.

Ronald Sandoval: Sandoval was already working as Da'an's protector when Boone joined him. Sandoval had been an FBI agent before the coming of the Taelons, and he was proud to be employed as the attache between the Humans and the American Companion. After all, Da'an was the most important Taelon on Earth and was also the Companion to the United Nations. However, Sandoval was a rather arrogant man who already believed himself to be above the common man. He was married to DeeDee, but after he obtained his CVI he saw her as a hinderance, distracting him and even threatening his position. He was afraid that he would reveal Taelon secrets, so he used his position to have her committed to a psychiatric facility and had her kept heavily drugged. It is later mentioned that he used this method of containment on other people as well. Sandoval was one of the first Humans to be given a Skrill, which was named Raven.

Sandoval changed in his attitude towards Da'an as he saw Boone taken into the Taelon's confidence. His own CVI was fully-functioning, and he was unable to divorce his complete obedience to Taelon interests with his Humanity. But he was puzzled and irritated by Boone's constant questioning of Da'an and the Companion's tolerance of this. His devotion to Da'an began to diminish, and by the end of the first season, he had switched allegiance to Zo'or, who was now Companion to the United Nations and head of the Taelon Synod.

Sandoval was more than merely a devoted slave of the Taelons. His CVI began to deteriorate and regaining his free will sent him into a frenzied race to regain his life. He rescued DeeDee and hid with her until she was free of the drugs that kept her catatonic. He had no intention of returning to his life as a Taelon protector, however his broken CVI would eventually kill him. So when Boone captured him, there was no other choice but to have a new one implanted. Sandoval had been one of the first recipients of the original CVI, which had contained information about the Taelon mission on Earth, but his second CVI did not. Before he became completely obedient again, Sandoval told Boone that Humanity needed the Taelons as much as the Taelons needed Humanity, though he no longer knew why.

In the alternate universe, Sandoval was named Jason and he was the leader of the Human resistance against the Taelons. He was married to Maiya, who travelled to Ronald Sandoval's universe. She was unable to resist confronting him, though she was bitterly disappointed to realise that he was working for the Taelons. Sandoval became obsessed with her, although as Lili pointed out, his CVI should have prevented that.

Sandoval is more complex than his role as Boone and Liam's counterpart would appear. Despite his obedience to the Taelons, he is capable of being politially ambitious. His change of allegiance to Zo'or is motivated by wanting to be on the "winning" side, and he believes Da'an to no longer be an authoritative figure. He is the only character who is credited in every episode of all five season. At the end Sandoval has changed allegiance again, to Howlyn, the leader of the Atavus (Taelon/Jaridian hybrids).