Before Dan(ielle)could say anything, Kat grabbed her by the wrist and dragged her upstairs to her room. Kat opened her closet door and fished out a number of packages.
"Mom and I picked these out when we started planning for you to become my familiar. We kept them pretty plain; we figured it'll be a while before you're accustomed enough to girls' clothing to want to wear anything overly feminine." She opened some of the packages and pulled out a white blouse, a black knee-length skirt, and a plain white bra and pair of panties.
"W-wait a minute," Danielle sputtered. "I'm not wearing that...stuff!"
Kat rolled her eyes. "Oh, come on," she said. "We already established that you're going to be this way the rest of your life. Familiarhood is not a temporary thing. It's not even a temporal thing; you'll find that everyone outside of your family now remembers you always being a girl. And besides, it's going to take a lot more seamstressing skill than you currently have for you to be able to wear pants, with that tail of yours. Now while you try to get these on, I'll explain how the whole 'familiar' business works in between laughing at your ineptitude."
Resigned to her fate, at least for the moment, Danielle picked up the panties and began threading her tail through the hole that had been cut in them. "Now the first thing you'll notice," Kat began, "is that people won't notice that you're an anthropomorphic cat unless they already now or you actively want them to know. That's just one of your abilities. They'll see you with only the cat-ears and tail, and your human image's canine teeth will be more pronounced. And they won't think anything of what they do see; most likely they'll rationalize it as cosplay, although some of the geekier types might think 'huh, catgirl' and not find it unusual. Now as to your, no, no, that's not how you put on a bra!"
Kat took a moment to help Danielle put on the bra. Danielle had been a little weirded out by how it felt to have her panties so snug without her privates feeling squashed the way they would have before, but that was nothing compared to how it felt to have her new breasts hugged up to her chest by the bra as Kat helped her fasten it around the back, continuing her lecture.
"As I was saying, your powers as a familiar are primarily passive. Your job is mostly to simply exist; you just channel mana (magical energy, if you were wondering) for me simply by being around. The support you provide does drop proportionally with distance, but it's inverse-of-distance, not inverse-of-the-square-of-distance like most things, e.g. gravity, so it's not such a big concern until you get over into another state or so. Hey, don't just stand there, get dressed!"
Danielle snapped out of the daze she'd been in from the new sensations coming from her now-clothed breasts, and hastily picked up the blouse and began to put it on as Kat continued.
"Anyway, that's your main job, providing extra mana flow for me. You can also take advantage of the mana flow you provide to do some simple magic of your own; Mom said she'd be happy to teach some utility spells. However, you won't be able to produce really complex effects like bodily transformation, so don't get any ideas about changing yourself back. I'd just redo the effects anyway. Also, I don't want you leeching all of my mana, capisce?"
Danielle nodded as she finished putting on the blouse and picked up the skirt.
"That's really about it," Kat continued. "Of course, there's other issues like helping you adjust to being a girl, but both Mom and I will help you with any questions you have. Brief rundown: you're fully female in body, i.e. able to reproduce (and yes, you're sexually compatible with normal humans; your children will be cat-people like yourself, though they won't have any magical ability, and hence won't be disguised; that'll totally blow your cover, so think long and hard before doing anything that might get you pregnant.) You have a menstrual cycle, like any of us. Don't grimace like that; the other option was for you to have a heat cycle like a full cat does, and I'm entirely sure you don't want that. Given time, you'll become fully female in mind, although that doesn't mean what it stereotypically is supposed to; there are some mental adjustments, like becoming reoriented to be attracted to the opposite sex (you were straight to begin with, right? I thought so,) but you'll largely remain yourself. Oh good, you're dressed. Come on and take a look." She grabbed Danielle and dragged her into the bathroom, positioning her in front of the full-length mirror.
A long, soft gasp escaped from Danielle as she surveyed the image the mirror showed. Looking back at her was a creature of soft beauty, despite the feline appearance. The figure was softly curved, with a pleasant but not oversized bosom. The legs were shapely and beautiful, but not laughably long. The hands, despite the claws and pads, were slender and had a delicate appearance, although Danielle could feel that all the parts of her body were much sturdier than they looked. The face, despite its muzzle shape, was still pleasantly soft, and was perfectly framed by the long, silky brown hair that cascaded down past the shoulders. The eyes were human, brown and beautiful.
"Yeah," Kat said, "you came out pretty darn good, if I do say so myself. Now c'mon, we gotta go introduce you to Mom, Danielle!"