- "Only prepubescent brains are malleable enough to receive and deliver messages without damage."
A messenger is a person who is taken over for a short time in order to relay a message from the future. Messengers are usually people who are prepubescent, since—as Marcy explains after we see the first messenger—only prepubescent brains are malleable enough to receive messages. Postpubescent messengers are possible, though the messenger dies. It is impossible to send a messenger further back in time than the arrival point of the most recent Traveler or messenger.
In the second season, The Director used Oliver Norton and Irene Ingram as messengers to order Vincent Ingram to self-terminate. After Protocol 3 was suspended for members of the Faction, Faction members were routinely killed by messenger. In the episode “Traveler 0027”, the Director is shown having a conversation with a Traveler in real time. Elderly men and women on life support are used as messengers, with each presumably dying after saying a few sentences.