In the aftermath of taking down a hydra station, Doctor Strange finds a frightened girl hiding amongst the rubble. When Theo sees him, she gets a fright and sends a burst of electrical energy towards him. After he avoids the blast he talks to the 8 year old, he learns of her electrokinesis abilities and how she doesn't know how to control them.
After a while of getting to know each other Stephen decides to take the young girl in and later adopts her.
Theo is taught how to control her powers with help from her adoptive dad and Wong and spends a lot of her time at Kamar-Taj training with others.
Quite a few years later, a girl from another universe appears and things become very chaotic...
(Electrokinesis- the power to manipulate, generate and absorb electrical energy.)