Although she goes by either "Meg" or "Meg Tesseract" or "Tessa", Mender Tesseract follows one of the first rules of time travel in that you do not give out your real name. Thus we have no idea who she was prior to calling herself "Tesseract", named after a hypothetical four-dimensional cube.
As with some of the other questionable Menders, Tesseract is a villain with a history of infamous time-related experiments conducted on people with or without their knowledge or permission.
One of her earlier infamous experiments involved the use of a temporal anchor. She hired Agent Shadow of Shadowborn to kill computer consultant Sally Bright, who was about to become a tech expert for the Guardians of the Dawn superhero group. Agent Shadow followed her orders and shot Sally Bright with several rounds, with the "kill shot" being a special round which contained a temporal anchor. This placed Sally's now deceased body in a state of temporal stasis, which would allow Tesseract to have an unrestricted chronicle of all events that occurred from that moment forward.
The experiment ended when the Kheldian being Talmyia felt the "call for justice" and merged himself with Sally Bright's body, transforming them both into the Peacebringer Redeemer Daybright. The information collection was put to an end once that happened. The data was then removed from the anchor by Doc Tor, who considered what Tesseract did to Daybright to be an abomination.
According to Galatea Powers, Tesseract said she was forced to abandon all of her earlier time-related experiments as a condition of being a mender. But that does not mean that she is done manipulating events in time.
Indeed, that is why she was chosen to be a Mender in the first place.
Powers and Abilities
As with the other Menders, Tesseract's abilities are a complete mystery. As a Mender, she has access to all of the secrets of Ouroboros, most of which include time-based weapons.
The two things about her that are not a mystery is that she is very intelligent and also very good at manipulating people to do her bidding.
Mender Tesseract is extremely arrogant. She sees herself as better than everyone else, including Mender Silos. If not for their rules, it is believed that she would have enslaved our time period long ago and rule all eternity as an empress.
Her arrogance is such that she does not have to lie to people. She feels she has nothing to hide or conceal. She considers mastery of time and space to be an entitlement for herself and the other Menders. She tolerates the "rules" of Ouroboros only because it gives her license to do anything else she wishes.
Tesseract does seem to consider both Doc Tor and Galatea Powers to be threats to her operations. For Doc Tor, it is because he is a known time manipulator. But for Galatea, it is because she has the potential to become not only an Omega Incarnate, but, in one possible future, she becomes a Mender herself.
(Please see her Paragon Wiki entry for more information.)