Alice: Former girlfriend of Michael Vaughn.
Dr. Judy Barnett: As the CIA psychiatrist, Dr. Barnett is entrusted with much information in her sessions with the agents. In season three, she had a brief affair with Arvin Sloane.
Dr. Edward Brazzel: A researcher who invented a non-invasive technique to treat long-term amnesia. (The technique involved using dreams to access memories.)
Thomas Brill: Acquaintance of Jack Bristow. Also knew Vaughn's father.
Jack D. Bristow: Sydney's father. Was a double agent for SD-6 and the CIA.
Sydney A. Bristow: Former college student; has since graduated. Worked as an agent for the organization SD-6 for seven years until she learned that they were not part of the CIA as they claimed. She joined the real CIA and continued as a double agent with SD-6, ultimately achieving her goal of bringing them down.
Francine 'Francie' Calfo: Sydney's roommate and long-time best friend. Francie was murdered by her genetic double, who then took Francie's place as part of Sloane's mysterious Phase Two.
Elsa Caplan: Wife of Neil Caplan. Also an SVR agent, secretly reporting information about her husband to the Russians.
Neil Caplan: Was recruited out of Cal-tech by the NSA. He knew that his wife was a spy, but she didn't know that he knew.
MacKenas Cole: Led an attack on SD-6 in season one and was captured by the CIA. Later he somehow achieved his freedom and became one of the leaders of The Covenant.
Conrad: A monk who knew much about the works of Milo Rambaldi. He was kidnapped by the Covenant, but died as Sydney and Vaughn attempted to rescue him. With his dying breath, he told Sydney that The Passenger is her sister.
Irina Derevko: aka "The Man"/Laura Bristow. Sydney's mother. Was a KGB agent, assigned to marry Jack Bristow and get information from him. When Sydney was six, Irina faked her death and escaped. Years later, Sydney was stunned to find her mother alive and apparently working for the bad guys. Later, Irina tried to prove that she wasn't bad: her true allegiance remains uncertain.
Katya Derevko: Sister of Irina Derevko. Katya joined the Russian Secret Service for the sole purpose of helping Irina find her younger daughter, Nadia Santos.
In season three Katya appeared to be helping Sydney, but that turned out to be a lie.
Di Regno: A man whose DNA profile was sketched by Milo Rambaldi five hundred years ago. The man's DNA is the key to decoding a page of the Rambaldi manuscript that foretells dates of apocalyptic events. Di Regno's heart is one of the parts of Rambaldi's Il Dire machine.
Marcus R. Dixon: Was Sydney's partner at SD-6; when she told him the truth about SD-6, he was instrumental in bringing them down. Later he joined the CIA and was promoted to management.
Allison Georgia Doren: aka A.G. Doren. Was one of the children involved in Project Christmas. Allison later allowed herself to be genetically altered to look exactly like Francie Calfo. Allison murdered Francie and took her place; using her relationships with Sydney and Will Tippin, Allison gave Sloane information about them. Allison was last seen bleeding on the ground after being stabbed by Will.
Ana Espinosa: An agent of K-Directorate. In Season One, Ana was Sydney's nemesis, and they clashed frequently as they took on missions to retrieve the same items.
Marshall J. Flinkman: A former agent of SD-6, Marshall is a super-genius who makes gadgets that the agents used. After the fall of SD-6, Marshall was recruited by the CIA, where he continues to save the day.
Daniel "Danny" Hecht: Sydney's fiance. He was murdered after Sydney confided that she was really a spy; Sydney's employer considered this a breach of security. It was Danny's death that showed Sydney that SD-6 was not what it seemed.
Noah Hicks: An ex-lover of Sydney's and a fellow SD-6 agent, from years ago. They ran into each other on a mission, and started reconnecting. However, Sydney soon discovered that Noah was also living a double life, as an assassin named The Snowman. Hicks was last seen dying after a knife fight with Sydney, who didn't realize that the masked man she was fighting was actually Noah.
Kendall: Was an officer of the FBI. Later revealed to Sydney that he secretly worked with the Department of Special Research, which is very interested in Rambaldi's works. When Sydney escaped from The Covenant, she went to Kendall.
Andrian Lazarey: A Russian diplomat. Father of Julian Sark. Lazarey was a friend to Sydney while she was posing as Julia Thorne; he and Sydney searched for clues to one of Rambaldi's mysteries. Later, Sark ordered his father to be killed before he could give the CIA any information.
Dr. Jong Lee: Sometimes referred to as the Sadistic Dentist of Asian Persuasion. Dr. Lee is an expert at torture, and he has used his skills on Sydney, Will Tippin, and Marshall Flinkman, to name a few. The tables were turned in season three as an enraged Vaughn tortured the doctor in an effort to gain information.
Senior Director Robert Lindsey: Head of the National Security Council. Was the NSC boss of Lauren Reed. Lindsey was later revealed to be a bad guy when he tried to blackmail Sloane into having Sydney murdered. Unfortunately, Sloane hired a man to take out Lindsey instead.
Leonid Lisenker: A top Covenant official who defected, claiming that "what they are planning, it keeps me awake at night."
Milo Rambaldi: Was Pope Alexander VI's chief architect. He was an artist, an inventor. His designs were so technologically advanced, at the time, they thought he was heretic. And he was executed. Now, five hundred years later, many believe he was a prophet. And SD-6, CIA, GRU, K-Directorate, the entire intelligence world, is on a Rambaldi scavenger hunt. Why? They assume that Rambaldi had some sort of master plan.
Lauren Reed: Wife of Michael Vaughn. Former employee of the National Security Council. According to Sloane, Lauren wanted to be a field agent, but her father prevented it, thinking it too dangerous for her. Later, it was revealed that Lauren was working for The Covenant, and she married Vaughn only to acquire information from him. Lauren was last seen having been shot by Vaughn, who was trying to save Sydney.
Olivia Reed: Mother of Lauren Reed. Olivia Reed was also an agent of The Covenant. She murdered her husband when her employers ordered it.
Senator Reed: Father of Lauren Reed. He was heartbroken when Jack Bristow presented information to him that proved Lauren was really working for the bad guys. Senator Reed approached Lauren, offering to cover for her. He was murdered by his wife, who framed him as the mole.
Nadia Santos: aka The Passenger. Milo Rambaldi spoke of her in his prophecies. Nadia is the daughter of Arvin Sloane and Irina Derevko. Nadia grew up in an orphanage; Sydney found her working as an agent.
Julian Sark: Young, handsome and deadly, Sark works for The Covenant, where he planned a coup and became co-head of the organization's North American operations with Lauren Reed. When captured by the CIA (and threatened with torture) Sark's loyalties become quite flexible, and he'll sell out other people to save his own skin.
Arvin Sloane: The former director of SD-6. Sloane recruited Sydney into SD-6, and according to Jack Bristow, this was when their long-time friendship ended. Sloane is obsessed with the works of Milo Rambaldi. Sloane claims that learning he had a daughter made him change his evil ways.
Will Tippin: Sydney's friend. Will worked as a reporter, but curiosity almost killed the cat when he learned too much about SD-6 while investigating the death of Danny Hecht. Later, Will unknowingly began a relationship with friend Francie's evil double, and again he was almost killed. For his safety, he was sent to the Witness Protection Program.
Michael Vaughn: Vaughn was assigned to be Sydney's handler, or contact, when she first joined the CIA. As they worked together, their mutual attraction grew. After the fall of SD-6, Sydney and Vaughn began to date. However, after she disappeared - at the hands of the evil Covenant organization - Michael fell in love with and married Lauren Reed. Later, Lauren was revealed to be working for the Covenant, and Vaughn shot her (repeatedly) to save Sydney.
Eric Weiss: A CIA agent. Also a good friend to Vaughn and to Sydney.
