Humanity needs to fight again giant organism known as Kyoryu. Children operate robots known as FRANKXX. Code Number 016 Hiro was a child prodigy but due to a failure is being handled as non-existing, till one day he meets Zero Two. She turns him her co-pilot.
It sounds like she sucks his energies/blood, and because of the good balance they are able to show FRANXX final form. I'm not against meccha, and can even understand the fanservice at parts - but not all. Also didn't like the meccha desing.