Frill is an artificial human created by Acca and Ura-Acca.

She was designed by Acca and Ura-Acca to feature traits a father would want to see in their daughter. They designed her to be cheerful and clever, showing intelligence much higher than that of the average fourteen year old. They also gave her a sense of humor.

Acca and Ura-Acca also designed her with flaws, giving her the capacity to express negative emotions and selfishness. Frill is shown to be playful around Acca and Ura-Acca, viewing the three of them as a family.

(Source: Wonder Egg Priority Wiki)