The choice of a young girl protagonist in Drill Dozer was decided by Ken Sugimori as he felt the juxtaposition of a "frail" character would be more interesting than the sort of masculine character that would typically be depicted in mech-suit and vehicle-themed media. Concept art shows a tokusatsu-style, presumably-male character in the same vein as Game Freak's previous title Pulseman with a drill for an arm, before it was chosen to have a character piloting a mech instead.

Character designer Hironobu Yoshida was opposed to this decision because he found women to be hard to draw, and added thick, boyish eyebrows to Jill's design as a subtle form of protest.

Jill's noticeably exaggerated hairstyle was derived from the appearance of a roll cake, when Yoshida was eating one at home. He asked his wife to prepare two more roll cakes and placed them atop a sketch of Jill's concept art. Yoshida noted that other characters in the game were based on foods, with the fish boss Fuguru being based on a fugu fish, and one unspecified character being designed to appear like an onion (likely Gearmo, whose long white hair strongly resembles a negi, an onion native to China that has been cultivated in Japan).