Mantis, also known as Vivian Rutledge is a member of The Messengers, sent to battle Cole and Ramse for control of the timeline. She was genetically engineered in 2015 using Olivia's ova. After being sent back in time, her own ova were used to genetically engineer Olivia herself, creating a never-ending loop. In her later life, she found herself doubting the Witness and his vision.
Mantis and the eleven other Messengers are born on November 11, 2015.
After securing the time machine, Mantis and 5 other Messengers are sent back in time.
Mantis and an unnamed male Messenger arrive in 1944 from 2043. She kills and impersonates a woman, Glennis Applebaum, in order to get closer to their target, Thomas Crawford. Mantis seduces Crawford and then kills him in ritual fashion with her bone knife, but to her dismay, the expected result does not occur. She and her partner soon learn that they are looking for Crawford's son, Tommy Jr. 
At the mental institution where Tommy is a patient, she runs into Ramsé, who had splintered from 2044 to save Cole and Cassie. At first, he is able to mislead Mantis that he is still working for the Army; she laments that, unlike him, she is destined never to have children. When he claims The Witness asked him to come, she realizes he is lying and handcuffs him to a radiator. 
Later Mantis and her partner surprise Cole and Cassie, who are also looking for Tommy. She kidnaps Cole and tortures him with electroshock to make Railly give up Tommy to her. She performs the ritual again and kills Tommy with the bone knife (which is carved from Tommy's own breastbone), causing a temporal paradox that knocks her out. Cole and Robert Gale come to interrogate her in the hospital. She declares that the Witness wants to end "time's cruel destruction" by breaking time itself, and that she wasn't expecting to survive the paradox. She overpowers Cole and Robert and escapes. Years later, Mantis gives birth to the Pallid Man. He is there at her death bed in 1971. 
In the intervening years between 1944 and her death in 1971, Mantis's ova are used by Albert Kirschner to genetically engineer a child: Olivia. In 1961, Mantis leads a group of Army members in an attack on Dr. Kirschner's laboratory in order to capture the child and rescue her from Cole, Cassie, and Ramse. They succeed in fighting off their enemies and take Olivia back to America. There, Mantis inducts Olivia into the Army of the 12 Monkeys and promises that she will soon have a big brother to watch over her.
By 1953, Mantis has a son, the Pallid Man, and a husband, Zalmon Shaw. Shaw is considered to be a missionary for the Army of the 12 Monkeys. Together, the family travels around to visit towns where tragedies have struck. They prey on the victims' families, promising them reunions with their loved ones in an eternal forest of red. One of their missions is infiltrated by Cassie, Cole, and Robert Gale. The three are able to escape as Athan Cole and his Guardians gas the attendees inside the revival tent.
By 1971, Olivia is a teenager, having been brought to America to be raised by Mantis. She's run away from Mantis and is living with other runaways, closely pursued by her mother. In an attempt to gain Cassie's sympathy, Olivia portrays Mantis as a radical believer who wants to take Olivia's child away, and that she's fled to protect her baby.
Later, when Cassie is confronted by Mantis, it's revealed that Olivia was in fact the radical believer and that she who ran away because Mantis wanted her to abandon the Army and the Witness. During the showdown between Cassie and Olivia, Mantis intercepts a bullet meant for her daughter, fatally wounding her. Her death scene with the Pallid Man shown in Season 2 immediately follows these events.
- Mantis has never been given a name on screen. She is referred to as Mantis in the scripts, as well as by the producers when they refer to her. IMDB lists her as "Vivian Rutledge."
- It's not shown when Mantis received instruction from the Army that would lead her down the path of working with Dr. Kirschner to create and raise Olivia and other Army initiatives. She had previously believed that her cycle and her role in it ended with the paradoxing of Tommy Jr. in 1944. It's possible that the Guardians sent back with Athan Cole put her on a new path.
- She refers to a "Father" who "engineered her too well."
It's unclear who this refers to.This most likely refers to Albert Kirschner.