Cheyenne Wyoming is a Vesper agent, who most recently held the title of Vesper Six. She has a twin brother, Casper, who is also a Vesper. She has a purple triceratops tattoo on her left wrist, identical to her twin brother's.
- Professional Stunt Double.
- Majored in Psychology at the University of Chicago.
- Speaks Italian.
- Master of Disguise.
Cheyenne distracts Jonah and Hamilton while Casper traps the Cahills who just found the map with a knife. Cheyenne and Casper later pursue the Cahills to the library Katja Mavel works at. Cheyenne holds Atticus although Jake tries to stop her. Cheyenne, along with her brother lock Dan and Amy in a sealed room where they are in danger of losing oxygen. Cheyenne and Casper flee after Dan and Amy escape.
Cheyenne and her brother stole "The Book of Ingenious Devices" from the Pergamon Museum while Amy and Dan are trying to steal the Golden Jubilee Diamond. Later she and Casper follow Amy and Dan to Timbuktu to find the "Apology". Cheyenne and Casper then earn the nickname 'Tweedledumb and Tweedledemented'.
In Day of Doom, Casper and Cheyenne transport the hostages to the Rocky Mountains in Colorado, and talk about betraying Vesper One, however, Sandy Bancroft, pretending to be asleep send a voice note to Vesper One, and Casper and Cheyenne are locked in a room with water tied to a pole, however Amy and Dan save them. After the Machina Fini Mundi is destroyed, Cheyenne is arrested with Casper, and Sandy Bancroft.
Cheyenne's parents are criminals, and now serving twenty-five years to life in prison (as stated in A King's Ransom).
- Casper Wyoming (twin brother)
- Mr. Wyoming (father)
- Mrs. Wyoming (mother)
Cheyenne has a talent for disguises, and has had 4 different aliases during the series.
- Gwynne Hyecimone (Vespers Rising Mission, and also an anagram of her name)
- Cassandra Prince (The Medusa Plot Mission)
- Vanessa Mallory (A King's Ransom, used to stay with Dan and Amy at a train)
- Nicole (A King's Ransom, used to try to kill Jonah and Hamilton)
- She appears for the first time in The Medusa Plot as the pretty biker working with him (thought that also could be Isabel Kabra), though her name is not mentioned.
- Like Casper, she also has a purple Triceratops tattoo on her wrist.
- After Casper failed his mission to get Gideon's Ring in Switzerland, Cheyenne replaced him as Vesper 6.
- She loves to tease Casper about his fear of bats,as shown in The Dead of Night.
- She has a metal rod in her wrist because her brother was not happy when he found out that she was replacing him, implying that he broke her wrist.
- She, Casper, and Sandy Bancroft are arrested by the authorities.
- She has a total of four aliases, making her the person with the most aliases in the Cahills vs. Vespers series.
- She could be based of of Damien Vesper's muscle, Craven.
- Cheyenne and Casper are cities in the U.S State Wyoming.