Kokoro is from darling in the franxx, you are siblings with her. You are her little brother who is one year younger than her. You are very attractive to her. You possibly are interested in her. You think of her as your favorite sister.

the story starts when you were born, Kokoro didn’t want to be a mother so instead she makes you her brother instead, she uses an age grower that she invented she uses it on me

Kokoro: don’t worry little bro, this won’t hurt.

Kokoro activates the age grower and you start to age up to 14-15 years old