Niffenegger creates characters that go beyond the flat and archetypal. The twins are intriguing copies of each other, making them similar yet different in distinguishably important ways. This saves them from becoming dull and further examples of overdone. They create a power play between themselves, not intentionally but due to their natures, and it's fascinating to watch them play out. The characters they meet in London are also anything but ordinary. Niffenegger really worked to explore character psyches and this shows through the character development. The reader can actually witness growth and change, allowing them to sincerely come to know these inhabitants of the building. The relationships that build between the twins and the Londoners are unusual but believable. It's not a real stretch to see watch them unfold and they don't bring up any serious questions of believability.
Throughout Her Fearful Symmetry are many different, interweaving story lines. Niffenegger does a good job at tying them together and to not leave any hanging for too long before returning. There's a well-thought-out balance between individual character's stories and the overarching story as a whole that offer many vantage points and points of view for the reader to consider. It keeps the novel interesting and helps flow it along. Where the novel falters is in the ending. After so many unusual and unique plot twists and extraordinary occurrences, the ending feels mildly anticlimactic and a little open. It certainly wasn't expected but it left me wanting more and feeling a little let down.
My only (minor) gripe with Her Fearful Symmetry is that, while the plot is mostly unpredictable and has the reader wanting to keep flipping pages, some points may be slightly challenging to grasp. If you are not a fan of ghost stories and cannot believe in such happenings, even in fiction, this book will be a little hard to digest. But if you can alter reality to accept the novel for what it is, you will enter a fascinating world centered around the London cemetery full of all sorts of ghosts.