What's the Name of That Book??? discussion

I cannot remember author, title or character names, but I can remember a good deal about the plot.

THE BOOK: followed two girls (late teens or young adults) and I believe was written from a split perspective between the two. One was pure evil and one was pure good (but it was a FANTASTIC book which dealt with this cliche marvellously). Both had powers such as super-strength, and later, flying. Both got their powers from their mothers - the good one inherited hers when her mother mysteriously died, the bad one after she killed her mother after pushing her off a cliff.

THE COVER: showed two girls facing each other in silhouette, and I think featured some colours coming out of their hair or something. There could have been purple involved.

KEY PLOT POINTS (spoilers, obviously):
- The good girl grew up in an orphanage or something. One time, she accidentally broke a bully's arm and felt really bad about it. Later (when grown up) she worked in a cafe, or a bookshop or something. At the end of the book, she was pregnant by her boyfriend. This made her extra powerful. OH and she fought crime and she had this really cool (implied) lesbian friend who helped her. I think the friend's name was Bryce? The bad girl killed Bryce though.

- The bad girl was a total BADASS. She also just killed people left, right and centre. She made herself like a gangster empire? And she had one friend, Liz the psychiatrist. Ultimately, she killed Liz - but she kept Liz's ear to talk to. That ear eventually like... rotted, so she ended up ripping the ears off a couple more psychiatrists. They 'weren't the same' though. Oh and initially, she killed her own mother (ruthlessly) to get superpowers. I think she pushed her car off a cliff.

- Both girls were immortal. Like, they could be killed (and both were, on multiple occasions), but they came back to life. They battle each other in the end, and the good one kills the bad one. Only, however, because the bad girl is so tired of her life, so she is willing to die. And then PLOT TWIST the good girl is pregnant with twins - a good one and a bad one.

- I think the whole thing had something to do with Celtic mythology. There were also talismans involved which increased their powers or something.

As you can see, it was a killer book and I really want to find it again.

Prophecy of the Sisters by Michelle Zink?

No, that's not it, sorry! They weren't sisters; they weren't by blood related in any way.

Could it be The Girl Who Would Be King? It's a very little-known book that I haven't read, but it sounds a lot like it.

The Girl Who Would Be King by Kelly Thompson for Elias' suggestion.

Yes, The Girl Who Would Be King is what I was looking for. Thank you so much!