At face value, Darling in the FranXX is a mech series that's full of fan service and follows a group of young pilots fighting to protect their mobile city from marauding kaijus.
However, there's a ton of mysterious elements lurking below the surface. Why are the pilots referred to as contaminated? Who does Zero Two seem to deadset on her belief that Hiro and the rest of his team are destined to die? With the series being an original story from A-1 Pictures and Trigger, there's no manga to provide spoilers or even any additional backstory. There's definitely some deeper enigma surrounding the fact that none of the teenage pilots ever reportedly reach adulthood other than the fact that their job is incredibly dangerous.
If the trailer for episode 11, titled "Partner Shuffle" is any indication, whatever is happening to Code:556 aka Kokoro might provide a fairly significant clue.