Greta (グレタ Gureta) is the adopted human daughter of Shibuya Yuuri and Wolfram von Bielefelt. Greta met and befriended Gegenhuber Grisela while he was imprisoned in her adoptive parent's castle and later helped him to escape.
Greta was originally a Princess of the fallen kingdom of Zorashia. Before the kingdom fell, her mother Izura sent Greta to her homeland, Svelera, as a hostage. However, Greta's life with her new adopted parents, the King and Queen of Svelera (her mother's brother and his wife) was bad and they didn't want to take care of her.
Greta plotted to kill the Maou with the help of Gegenhuber Grisela by posing as the Maou's illegitimate daughter. This was due to the fact that her uncaring foster parents hated the Maou and she believed she could earn their love by assassinating him. Not long after that, Yuuri forgives and adopts her. Wolfram follows suit, reasoning that as Yuuri's fiancé he is also Greta's father.
Differences in the anime
In the anime she does not go to study with Hyscliff but stays with Yuuri and Wolfram in the castle. Also, Zorashia seems to still exist and in one episode, a man from Zorashia comes to take Greta back to the country because it's in need of a ruler. He ends up leaving her in Shin Makoku because she is happy there.