In the episode "Fearful Symmetry," Galatea decides to kill those aware of her creation in positions to expose her, in part to draw Supergirl out. Supergirl and Galatea battle until Project Cadmus cuts short the fight by destroying the facility, to rein in Galatea and cover their involvement.
Galatea is badly injured in her first fight with Supergirl, and spends several months in an intensive care unit tended by Dr. Hamilton's team. After recovering, she was then upgraded through intensive training and a biological enhancement. Dr. Hamilton artificially aged her body to her physical prime to increase her combat effectiveness. At the time, Supergirl is somewhere between 18 & 20, her last appearance in the show was her 21st birthday, but the upgraded Galatea appears to be in her mid to late 20s. When preparing to leave she hugs Dr. Hamilton and calls him "Daddy."
Galatea returns in the episode "Flashpoint." After the Justice League is framed by Lex Luthor for firing on a city with their headquarters' main cannon, Amanda Waller orders Galatea to lead an army of Ultimen clones against the League (episode "Panic in the Sky"). Although Waller calls off the assault after learning the truth, Galatea ignores the order and attacks the satellite. She is defeated by Supergirl who pins her against a wall with an electrical cable from the Watchtower's nuclear generator, frying Galatea with the electrical output of a small city. Whether Galatea survives this attack is unknown.
Galatea's costume is a homage to Power Girl, although their origins and personalities are significantly different.