Theta's origin is unknown. In fact, nothing is known about her beyond her relationship with the psychotic metahuman Epsilon.
Major Story Arcs
Theta first appears while Argent and Epsilon are out shopping. Epsilon, having remembered that he had an "appointment" to make, convinces Argent to take a break from shopping and go to the meeting place: a posh Manhattan hotel. When he gets to the room, Theta is waiting for him, and pulls him inside and begins kissing him. Argent, confused (and jealous), speaks up. And when Epsilon introduces her, Theta's clothes transform into a costume and attacks Argent. While the two fight, Epsilon appears to have a fit of amnesia. When he realizes what is happening, he leaps into action, only to be blasted across the room. Angered, Epsilon manages to subdue Theta, and proceeds to beat answers out of her, until Argent stops him. She then uses the reprieve to leaps out of the window and fly away.
Argent, confused about Epsilon and his connection to Theta, manages to track her to her quaint suburban home. Arriving home moments after Argent breaks inside, she prepares to attack, but Argent calls a truce, saying she only wants information about Epsilon. Theta believes she is only jealous, but she quickly denies it. The two step outside to talk, but then she reveals she was only trying to get Argent away from the house so she could kill her without worrying about property damage. The two fight, but Argent wins by trapping her in a force field. Argent asks again for information, and Theta claims that Epsilon was hired by her father to take down the Titans; Epsilon was meant to infiltrate while Theta reported their progress. Argent, upset, leaves without disabling her force field, leaving her trapped for an unknown period of time.
Theta is next seen meeting Epsilon on a rooftop, still upset about the last time they saw each other. Epsilon then reveals why he was acting strangely. D.E.O. orphan Kevin Tanake, who had the power to project his mind into another person's body, possessed Epsilon's body, causing him to act out of character. Theta is unconvinced, and question him to reveal the truth. Eventually, he gets impatient, kisses her, and tosses her to the ground, which is enough to convince her. Happy to have her boyfriend back, she suggests that they run away somewhere, but Epsilon has only one thing in mind; getting revenge on Kevin for what he did.
The two then make a beeline towards the top-secret D.E.O orphanage Kevin was living in. The two then find him immediately and begin blasting everything between them and their target. Argent, Arsenal, Tempest, and D.E.O agent Dakota Jamison try to fend the two off, but they barely hold their own against the duo's raw power. Argent and Theta go at it again, but they blast each other with such intensity that they knock each other out. As Epsilon goes in for the kill, Kevin possesses and unconscious Argent, and uses her powers to blasts twice, with the second being at full power. Theta wakes up in time to see Epsilon go down, and she blasts Argent/Kevin, grabs Epsilon's seemingly dead body, then flies away. Their fates are unknown, as they are never seen again.
Powers and Abilities
Theta shares most of Epsilon's meta-abilities, including super-strength, high durability, concussive energy blasts, and the power of flight.