@chibi: ...Might be able to work with that.
Witches form when a Magical Girl accumulates a critical amount of Grief. "Grief" is probably best described as "lack of magic/hope", as it is used whenever an MG uses magic (even existing uses a small amount), and its rate of depletion accelerates when some kind of despair overtakes them (usually dependent on their wish and circumstance). They usually hide in otherwise-unseen Labyrinths, which are basically extra dimensions of acid trips.
Appearance and Abilities of both the Witch and their Familiars are also determined by their situation, and can vary by exact circumstances. Oktavia's, for example, is a mermaid (her general story is a parallel to The Little Mermaid - NOT the Disney version, though!) and is similar to her Magical Girl Appearance (swords, knight-like armor). Hey Labyrinth is modeled after either a concert hall (Kyousuke's a violinist) or a pop concert stage (with not!Hitomis as back-up dancers to her), and scrawled everywhere is "Look At Me!" (her downfall was because Kyousuke didn't notice her and she couldn't bring herself to confess her love).
The damned queen and the relentless knight.

You did good. Then again, it's pretty hard to be as annoying as TAR.
This is... a ton of things, guys. Can we not do a ton of things like this anymore? Maybe it's just the whole phone thing making this seem worse than it really is, but...
MY SOUL IS DARK BUT MY HAIR IS COLORFUL — Brahian Pokémon Alchemist