I think this could be plausible as a stunt, but most people don't have the skill it takes to do this and I designed this for a were-mockingbird, so it's in here.

Mimic Sound -1
This character, after being exposed to a sound enough, can mimic it almost perfectly. Using Deceit, this character may mimic any sound that they could justifiably hear often (A guy on the streets won't mimic a jaguar's roar, and a tribal native won't be able to do a police siren). Performance could be swapped for Deceit with justification (like if the character is a comedian who's really good at impersonations). This only applies to general sounds, not speech.

Mimic Voice -1: This character after hearing someone speak for a long enough amount of time (Not a few words, but maybe a short conversation) can mimic that person's voice using Deceit. Just like Mimic sound, this can be used with performance as well.

They're interesting powers (or stunts) for social builds. They have lots of uses too. Have to scare off that criminal? Try a police siren, or mimicing a police officer. Want to distract him so your buddy can knock him out? Mimic the sound of his phone ringing, or mimic a woman calling him over. Wanna scare him? Ever heard a good horror movie scream, and didn't that ghoul you met a few weeks ago have a really creepy voice?

If you take it from the Shadowcat example, any attack that is entirely physical and most energy-based attacks go through. Anything specifically designed to hit her, diseases, and mental attacks still hit.

I GM at my school every friday, and the players love it. However, most of them aren't what you would call roleplay-oriented. The main way for the party to solve any problem is to go and kill the source. Some of them take it all like a joke, too. I swear that if I introduced an object that is integral to the survival of the universe one of them would pee on it. Still, I get the sense that some of them want to learn how to roleplay... so how do I teach them?

I don't think Kerrigan counts as a Demon. She's tough though... if anyone's ever played Starcraft 2, it takes a well-protected choke point to defeat her, which doesn't kill her, and she leaves it in shambles.

I did have a nemesis like this for an earlier game I did. One of the players was a werewolf much like Tera West (Except he was just raised in the wild, not born as a wolf). So, I put in a super-powerful werewolf who's enslaved to his killer nature. I put him in charge of a large gang of criminals, of which he was the figurehead. We never managed to get to beating him, though.

I like this thread, and would like to pose a question here:
I like catches, and have my characters use them, but I think some catches should have an additional component that adds to the damage when done with that item. For instance, faeries have problems with cold iron. But it doesn't just circumvent their defenses, it injures them. If you threw a small piece of iron at a normal person, it does nothing. If you throw a small piece of iron at a fae, they freak out and get injured.
What do you think about this?

1. What is a non-combat interesting way to use earth magic?
I think that one could make a living out of molding crystals for use in jewelry using magic.

2. What is a single target interesting way to use earth magic?
Diamond bullets.

3. What is an area of effect interesting way to use earth magic?
Wave of mud.

4. What is an interesting way to block using earth magic?
A huge sandstorm blocking visual things.

5. What would be an interesting way to impede someones movement with earth magic?
Have a golem form out of the rock and grab them, or instant quicksand.

6.Can you think of anything else that would fit the Dresdenverse and be really neat in a game for earth magic.
Triggering a fault line to create a city-wide panic. Good for evildoers.

Check the Generic NPCs thread in the resource collection for some others.