Dan, Dave and David are general fourth participants.
Erin is a fifth participant.
Eve is an eavesdropper. She can listen to the messages exchanged between Alice and Bob, but she cannot modify them. That makes her a passive attacker.
Faythe is someone or an organization that can be trusted: for example a trusted advisor, a courier, an intermediary or a repository of key services.
Frank is a sixth participant.
Grace is a representant of the government who might want Alice and Bob to implement backdoors in their protocols or deliberatly try to weaken standards.
Heidi is a mischievous designer of cryptographic standards.
Mallory (aka Mallet) is a malicious, und unlike Eve an active, attacker who often performs man-in-the-middle-attacks. He modifies and substitutes message, or replays original messages.
Securing systems against Mallory is usually harder than to secure them against Eve.
Oscar is an opponent, perhaps without malicious intend.
Peggy (aka Pat) proves that intendend transactions actually took place.
Peggy is, together with Victor, also very active in zero-knowledge proofs.
Sybil is a pseudonym attacker with a large number of identities (for example to subvert a reputation system).
Trent (aka Ted) is a trusted arbitrator who acts as a neutral third party.
Trudy is an intruder.
Walter guards Alice and Bob (Warden).
Wendy is a whistleblower: an insider with privileged knowledge.
Then there is also Arthur who asks Merlin and Paul who asks Carole.