Omake Gif Anime - Darling in the FranXX - Episode 13 - Zero Two's First Sweet

In a series of flashbacks of recovered memories, we learned that Hiro was the source of Zero Two's sweet tooth. Or sweet pointy teeth, in her case.

Hiro appeared to be the origin of many of Zero Two's favorite things. He introduced her to licking skin, to being her "Darling," and showed her fish swimming in a pond.

The main reason Zero Two is so obsessed with becoming a human comes from a picture book given to her by a mysterious kind person. Its story is about a beast princess who becomes human to marry a human prince. Hiro became that prince to her young self, but they were recaptured and their memories were forcibly removed. This also explains Hiro's change in personality and why he forgot his promise to Mitusuru. The next dark half of this story is ready to start.