My concept for Soapbox chan:

No fox ears. Longer hair, and a few shades darker, with a smug tsundere gaze (deep symbolism for how little Alex wants Soapbox-chan to exist.)

Even though she's Pleroma-chan's younger sister, she's taller, and has bigger tits. Alpha-Stace poses and all. Ideally she'd be the optimistic type but Pleroma-chan already has that look, so you need a contrast.

I haven't figured out how to integrate the name into the character. Maybe she's preachy. Or maybe it would be more fun for her to have pictures of soap somewhere on her clothes.

Also maybe she has a pet micro-elephant that rides on her shoulder too, cause legacy reasons.

Any more thoughts and suggestions?