She is a servant of Galaxia, always seen accompanying Sailor Phi and Sailor Chi and, like them, is named after a Greek letter (Theta, written Θ). The three of them are referred to as the Galactica Troops.
Depending on the musical, Theta disguises herself as a student or a backup dancer for the Three Lights in order to observe the Sailor Senshi and eventually attacks them with her partners. Theta does not have an individual attack, instead only shouting Sailor Galactica Attack, and Galactica Cry alongside Phi and Chi. In Eien Densetsu (Kaiteiban) two members of the Galactica Troops use Galactica Twister and Dark Power. Theta is eventually killed by Galaxia herself, along with Phi and Chi, for being too weak.
Like the other musical-exclusive characters, nothing of her origin is revealed. The three of them have more than one uniform, dependent on the musical. In the Sailor Stars Musical she went by the name Shadow Bug. This bug theme was dropped for the revision. She did not appear in the 12th musical, which was also based on the Sailor Stars series - she and the others were replaced instead by the Shitennou.
In Sailor Moon Sailor Stars, Sailor Theta used the alias of Shadow Bug while working under MC Fly. However, in its revision, she was simply called Sailor Theta, who (along with Phi and Chi) disguised herself as a backup dancer for the Three Lights while on Earth..