When Zero Two goes on a rampage and takes Hiro with her, the consciousnesses and memories of the two are merged, and Hiro begins to remember forgotten events involving a younger, redder Zero Two, as if she was the key to unlocking his repressed memories.
The appearance of Zero Two in Hiro’s early life is a revelation to someone who has always asked questions and sought answers but received none, and named other children like Ichigo and Mitsuru so they could be people and not mere numbers.
Hiro is indeed quite “special”, and Dr. Franxx always wanted him that way, to see how someone like him would fare as a parasite. But that comes at the cost of Hiro discovering the existence of the little girl with horns.
Dr. Franxx is not painted in the best light here, as if there was ever a good light to in which paint him to begin with. Whatever he seeks to learn from the girl he calls a “specimen”, all that matters to Hiro is that this very different and amazing little girl is being hurt by the adults, and he’s not okay with that.
When the adults stonewall him, he searches for a way to get to her, casting aside all fear of punishment from the adults precisely because they’ve always told him he’s so special. As far as he knows, he’s supposed to rescue the red girl.
He does, and for a brief, beautiful few hours, but not much more, the two are blissful in their freedom and gratitude for one another. Hiro gives the girl a name—Zero Two—literally licks her wounds, and reads from her beloved picture book, the story in which just happens to mirror theirs precisely: a beast princess and a human prince falling in love, then losing each other in tragic storybook fashion.
Unfortunately, that’s how the story of young Hiro and Zero Two ends, with the adults tracking them down, capturing and separating them, and forcibly removing their memories.
But back in the present, the sad ending of that story has been usurped by the writing of new chapter, in which Hiro remembers Zero Two was the girl with the picture book. Not a monster, just a girl who just happened to have red skin and horns, and who, like him, needs friendship, family, and love.
At the same time, Zero Two remembers that Hiro isn’t just fodder to help her become more human. He’s her Darling from “back then” after all—her one and only Darling. Perhaps the two have turned the next corner in their always twisted, often tragic, yet occasionally joyous lives. One can hope.