Sonique is a mutant who has the ability to create various sonic effects through the combined action of her vocal cords and limited psionic powers which can only be used in conjunction with the sounds she produces. By focusing her sonic vibrations into an intense beam, she can shatter any known solid substance except adamantium and vibranium or at lower intensities to stun people into unconsciousness without doing them injury. Theresa can use the sonic vibrations of her scream to propel herself through the air by causing acoustic energy to flow behind her. She can also use her sonic powers as a form of sonar. While using her sonic powers Sonique also automatically and psionically creates a force field that protects her body from the destructive effects of the sound she creates. The force field only protects Siryn against her own sonic vibrations and nothing else and can only be activated when she is using her powers.

Her mutant power manifested itself during the Theatre Troupe's raid on a train that was going to ship off humans for genetic testing. She first used her sonic lance to protect one of her rescuers, Nate Grey, from an unseen attacker behind him. After the success of the rescue, Theresa joined the Theatre Troupe and bonded with Nate. One morning at dawn while everyone else was still asleep, Theresa caught Nate as he was about to venture off on a search for answers on the astral plane and forced the telepath to take her along with him. While they were away, Domino and her pack of assassins had ambushed the remainder of their camp, claiming the lives of both Mastermind and Toad.

After the defeat of Domino's band, Essex, who revealed himself to be Sinister, killed Nate's adoptive father Forge. Enraged, he took on the Horseman mano-a-mano until Soaron and Sonique appeared on location to hold off Sinister so that Nate could escape, but they only complicated things further. Despite the distraction, Nate was able to defeat Sinister in the end and decided to hunt down Apocalypse alone. He charged Soaron with the care of Sonique and the two went off into hiding.

They were later re-discovered by Sinister who forced Jean Grey to brainwash them into becoming members of his Sinister Six, a group meant to aid Sinister in destroying the X-Men. When the Sinister Six met the X-Men in battle, Sonique was quickly overwhelmed by the combined efforts of Gambit, Nightcrawler, and the Silver Samurai. Following the Sinister Six's defeat, Psylocke was able to undo the mind control performed on Theresa. It is unknown what became of her after this.