According to director Hidetaka Miyazaki, Gwynevere's character design was inspired by a group of female characters from the 1974 Fujiko F. Fujio one-shot manga "Yasuragi no Yakata", about a stressed CEO who is directed to a members-only club by his doctor where men are dressed in children's clothes and cared for by giant motherly women who treat the men like infants as they regress further into baby-like states.

Gwynevere's extremely large breasts were added by the artist who designed her, who was reportedly so proud of his work that despite Miyazaki's standards, he did not have the heart to tell him to change the design.

Miyazaki also originally wanted to include a mouth on Gwynevere's palm and created animations for it, but this element was scrapped from her design.