Make her breasts allergic to clothing

Curious to see if the Ruby can alter more than just her appearance, you wait until Erica starts blowing on her coffee, then focus on her again and say, "Your breasts, and especially your nipples, are now allergic to clothing. The longer you wear clothing over them, the worse they will itch. Bras make them itch twice as much."

You don't notice any immediate changes come over Erica. Still, the hair color change wasn't instantaneous either, so you resolve to be at least a little bit patient. Erica takes a sip of her coffee, then looks up and asks you, "So, what made you decide to come over? Other than boredom, of course."

"Really, that's it," you say, watching your friend intently, "I just figured the two of us could hang here for a bit, maybe watch a movie or play video games."

"Yeah, that sounds like fun," she says, and you notice her squirm a bit in her chair, "I'm totally down for a couple rounds of Mortal Kombat if you're interested in getting your ass kicked."

"Are you sure that's what will happen?" you ask with a smirk, "Because if I remember correctly, I won best 3 out of 5 last time we played."

"I was just having an off day," she said, moving her upper arms closer to the sides of her chest and squeezing her sizable boobs together, "I just...." she squirms in her chair a bit more, "I'll have my head more in the game this time."

"Sure you will," you say with a dose of sarcasm, taking a sip of coffee to try and hide your growing smile. Erica's starting to look uncomfortable now. She sets down her mug and crosses her arms over her chest, a gesture you notice is meant to try and make it less obvious that she's scratching the sides of her breasts.

You set down your mug and do your best to look concerned, "Hey, is something wrong? You don't look so good."

"Yeah, I'm fine," Erica says unconvincingly, "The coffee's just a little hot is all. I'm letting it cool."

"While grabbing your boobs?" you ask, watching her face grow red in embarrassment and her boobs jiggle a little more from her itching, "Come on, what's really going on?"

Erica is quiet for a couple seconds, then sheepishly admits, "My chest is itchy."

"Itchy?" you ask, raising a brow.

"Yeah, and it's starting to dive me crazy!" she responds, not even bothering to be subtle anymore as she scratches the front of her chest, "I don't know what the hell is going on with me today!"

"Hey hey hey, calm down," you say, setting your mug down and leaning forward, "You might just be having an allergic reaction to something."

"An allergic reaction?" she says, looking down at her chest, "But the rest of me feels just fine. It's just my breasts that feel like this. And..." her face goes red as her voice trails off.

"And?" you ask.

"...My nipples," she grumbles, scratching the front of her chest with her thumbs, "There, you happy? My nipples are itching even worse than everything else!"

"It might be your bra then," you say to her calmly, "Or at least the detergent you used when you washed it last, anyway. Even if you didn't change it, allergies can appear and disappear over time."

"I know that," she says irritably, her itching starting to get faster, "I didn't... I didn't think it could flare up so quickly though."

"Guess it can," you say with a frown, "Either way, you're looking miserable. You should probably change into something else."

"Yeah, you're right," Erica says, standing up, "While I do that, take our coffee in front of the TV and set up the game, will you? I need something to get my mind off of this."

"Sounds like a plan," you say, reaching over to grab her cup before getting up yourself. Without so much as a thank you, Erica quickly goes to her bedroom and shuts the door. Once she's out of sight, you break out into the grin you've been holding back up until now. This was so awesome! You really could mold Erica's body anyway you wanted to with just a word! You giddily go over to the entertainment center and start setting up the game, thinking about your next move.

A few minutes later, Erica comes back out wearing a different shirt than before. She flops down onto the sofa and asks, "So, you ready to get schooled?"

"As long as you're okay to play," you reply, sitting down next to her and handing her a controller. You can't help but notice her nipples tenting slightly under her shirt; apparently, she couldn't bring herself to put on another bra.

"Don't worry, this is a dirty shirt," she tells you, "I haven't washed it in a while, so the detergent shouldn't be on there anymore."

Even as she says that though, you notice her hand idly reach up and scratch the side of her chest again. You doubt it will be too long before she's back to itching her breasts like crazy again. You think that you might have gone a bit too far with this change, and it might be better to undo it now and try messing with her in a less cruel way. Although... a darker part of you wants to push things even further. Like Mr. Light said, the power is yours to use as you see fit, and you can always change her back if something goes wrong. The choice is yours.