She needs them after becoming a human

Penny Polendina was always interested in being a real person, and thanks to the Relic of Creation, she was transformed from an android to a human girl. It was a miraculous transformation... that was cut short by Penny having an accident in her new panties for the first time.

It turned out Penny needed to learn certain bodily funtions like eating and using the bathroom as if she was an actual child, which was why the latest Winter Maiden needed to wear diapers now.

After resolving the conflict with Cinder and Dr. Watts, Ruby and Penny were enjoying a day out on the town, though Ruby had to carry a diaper bag for Penny for public outings. After getting some ice cream, Ruby felt Penny tapping her shoulder.

"Ruby, I feel a strange sensation in my abdomen. Is this a sign I need to use the potty?" Penny asked innocently.

"Huh?" Ruby was surprised by Penny picking up when she needed to go, but didn't want to question it. "Y-Yeah, sure. Let's find a bathroom for you." Ruby smiled a little as she took Penny hand and led her to the bathroom. Maybe she would start wearing panties for real soon enough.

"Oh, nevermind. It's gone now." Penny said in a very casual tone as a foul smell started to come from her, causing Ruby to sigh and check Penny's diaper. Unsurprisingly, there was a warm lump in the seat of it.

"Let's get you changed, Penny..." Ruby said as she lead Penny to a nearby bathroom while Penny waddled after her.