9'ζ, η, θ

9'ζ, 9'η, and 9'θ are characters in DARLING in the FRANXX.

Appearance
Zeta and her identical pistils (Eta and Theta) are triplets that have white hair, grey eyes, and silver masks. They are the third shortest members of the squad (the shorter ones being Alpha and Delta, respectively). Along with their teammates, the triplets sport the Nines' white-and-gold-emblazoned uniforms.

Personality
Very little is known of the triplets' personalities, other than that they are of one mind with their leader, Alpha. This compliance is manifested in Episode 18, where one of them joins in the lopsided hand-to-hand fight against Zero Two.

Episode 12: The Garden Where It All Began
9'ζ, 9'η, and 9'θ are seen with the rest of the 9's at the garden for maintenance. There, they bump into Squad 13 and former squadmate Zero Two.

Episode 15: The Bird that Shares Wings
9'ζ, 9'η, and 9'θ fight at the Gran Crevasse alongside the 9's and other squads from different plantations. Despite being highly skilled and the klaxosaurs mostly being Conrad-class, 9'ζ, 9'η, 9'θ and their squadmates struggle against the sheer numbers of klaxosaurs.

Episode 18: When The Sakura Blooms
9'ζ, 9'η, 9'θ and the rest of the 9's team up with APE soldiers to take Mitsuru and Kokoro for indoctrination. As Mitsuru and Kokoro are having a wedding, something against regulation, the standard protocol is they must have their memories altered.

Either 9'ζ, 9'η or 9'θ joins in with their squadmates against a hand fight against Zero Two, who tries to foil their operation. Due to the 9's having the advantage of numbers, they easily overpower Zero Two. Mitsuru and Kokoro are then taken away to have their memories altered.

Episode 20: A New World
9'ζ, 9'η, and 9'θ fight against the klaxosaurs and then VIRM in the 2nd Battle of the Gran Crevasse.

Episode 23: DARLING in the FRANXX
9'ζ is implied to have died due to lack of maintenance following the battle. The other two return with Beta and Gamma to take part in the galactic battle with Squad 13 against VIRM. However, all the 9’s, including Eta and Theta, are killed during the galactic battle.

9's
They are very loyal to their comrades and leave all diplomatic decisions to their leader, Nine Alpha.

Etymology
Similarly to Zero Two, they received their names from Papa. Zeta, eta, and theta are letters in the Greek alphabet.

Trivia
9'ζ
 * In the anime, Zeta dies from lack of maintenance and the two others are killed by VIRM. In the manga, all three are killed by the Klaxosaur Princess.