Rarity - greed, Fluttershy - wrath, Pinkie - negativity, AJ - fraud. Dash - disloyaly, refused to be in the drawing and abandoned her friends. Twilight gave up on her corrupt friends and left to someplace far, far away..
If you do, use the kinds that are in pencil shape. That's the easiest kind to use, as long as you know how to sharpen them (be ridiculously gentle). Whole sticks of charcoal will get powder everywhere- you clothes, your skin, your snacks, and long lasting black fingers.