The book (series) is
The Secrets of the Immortal Nicholas Flamel.
There are a lot of books that talk about things like this,
Yeah, it's a common fantasy trope.
where there are two people who are "chosen", and they're kind of opposites,
The Twins of Legend: silver and gold; one to save the world, one to destroy it.
So I guess that opposites is actually a god thing to write a book about.
Some Ideas Never Fail,
especially when it comes to writing.
More about how it's a common trope.
In this book, there's a
Aten was the overlord of Danu Talis, a tyrant in some ways, Macchiavelli's Elder master, but actually not a Dark Elder.
an insane old man,
and a boy,
who are opposites, but still work well together.
They stay together almost all the way through the books until the very end, when he destroys a world and she saves another. (Not too spoilery, I hope! It was certainly one of the best endings I've read - especially < SPOILER > Josh's transformation < /SPOILER > - although the books did lack something in consistency.)
I actually guessed beforehand that this was going be the subject of the OP's next riddle, having had a look at her profile page. This is the first puzzle on SE that I could have answered pre-emptively before it was even posted!