Zillah Green is probably the closest thing to a protagonist among the main characters of this novel, even though she is not present in all the important settings. She is the emotional center of the story, and the source of much of the considerable, creatively disruptive wild magic being referred to in its title.
Zillah is the younger sister of Amanda and the former lover of Mark Lister, both members of the Inner Ring of witchcraft in the British Isles. Her sister recognized very early on that Zillah had loads of magical potential, and urged her to train it, but Zillah tended to acquiesce, join one group or another, and then somehow slide out when the training was still very sketchy. It was as if she found she didn't want to be pinned down to any traditional type of discipline.
When the novel begins, she has brought her young son Marcus back home and is living in her sister's household -- helping take care of her laundry, her teenaged children, and her husband's laundry and shopping while Amanda tended to all her duties in the Ring. Zillah is very close-lipped about Marcus' parentage, although Auntie Gladys has always suspected that Mark Lister is the father, though she doesn't say so to anyone.
Zillah's been separated from Mark for over two years -- she broke off the relationship after seeing a true vision of Mark as a broken person, being sucked dry by some female and looking like someone trapped at the bottom of a well. That vision shocked Zillah so badly she couldn't stay with Mark any more, even though the pain of missing him is still driving her quietly crazy.
Finally, Zillah performs a homemade ritual on the spur of the moment that gets her and Marcus pulled along with a group of Earth magicians on a death mission to another world to combat pirates. In that other world, Zillah makes friends, confuses the pirates, and after a crisis occurs, escapes right into the hands of her worst enemy. It's an intricate, fascinating series of plots that in the end brings things crystal clear.