A brief description of the DARLING IN THE FRANXX manga:
The remnants of humanity live in mobile military cities, the plantations, where children are bred to fight the threat in gigantic mechas, the Franxx. The Franxx can only be controlled as boy/girl pairs – called parasites. Hiro, one of the children predicted to have a special talent, failed a test. So he cannot control any of these gigantic machines and lost all sense of life. When he stays away from the closing ceremony, he meets Zero Two, a notorious Franxx pilot who belongs to a race of people with Klaxosaur blood. After her partner is killed in a Klaxosaur attack, she convinces Hiro to become her new partner, whom she calls her Darling.