In the lore of Oz, Nessarose hasn't always had an actual name (like her older sister). She's the Wicked Witch of the East, and she's a pair of legs under a house wearing some snazzy shoes. Like her sister, we only have a few physical features, an interesting death (who else can boast of dying from having a house fall on them?), and a derisive name to go on. Upon entering the world of Wicked, we probably know less about the Witch of the East than we do about her infamous sister.
The mystery surrounding Nessa's character is emphasized in the book. We get a late introduction to her, and we hear little fragments about her before meeting her. She's shrouded in mystery, so that the eventual reveal that she has no arms is shocking. Nessa, however, is much more than a mysterious figure with unique physical traits and a dramatic death scene.
In a lot of ways, Nessa is the child of two fathers: her likely biological father, Turtle Heart, and the father who raised her, Frex. Nessa seems like Frex's mini-me, a fact that Elphaba notes frequently:
"Nessarose is a strong-willed semi-invalid," said Elphaba. "She's very smart, and thinks she is holy. She inherited my father's taste for religion. She isn't good at taking care of other people because she has never learned how to take care of herself." (22.214.171.124)
Nessa is actually so much like Frex that it's hard to spot any bit of Turtle Heart in her character, or her mother Melena for that matter. But before we declare a victory for "nurture" in the whole "nature vs. nurture" debate, it's worth noting that Turtle Heart is quite spiritual. He is a seer who believes firmly in "other worlds" and the presence of his ancestors. Nessa actually displays some mystic tendencies from time to time, as Glinda notes:
"It was especially moving – and irritating – to have to wipe away a trickle of tears brought on by some epiphany in Nessarose's rich inner spiritual life of which bystanders could have no inkling." (126.96.36.199)
And of course, she practices magic, whatever she wants to call it. Nessa has definitely taken in aspects from both of her fathers, which makes sense given Frex's assessment of Nessa's birth.
Nessarose was the proof of their brief union – theirs, and, obviously, Melena's as well. (188.8.131.52)
In a way Nessa epitomizes the book's theme of religion – she practically is religion. So aside from her two fathers, what exactly is the appeal of religion for Nessa? Why did she choose to make that the defining part of her identity?
Nessarose's decision to embrace religion so fully is probably related to two other key factors of her character: her lack of arms and her relationship with Frex.
First off, Nessa's lack of arms has made her life really difficult. This may be in part why she became extremely religious. She was never able to be independent, and when she gained independence later in life, with the help of Glinda's shoe-enhancing spell, the power went to Nessa's head.
But Nessa now thinks she needs no one, to help her stand or help her govern. She listens less than ever. In some ways I think those shoes are dangerous. (184.108.40.206)
Having independence thrust on her like that must have been dizzying for Nessa, and her ensuing power trip certainly created some problems (more on that in a bit), and she became more of an extremist than before.
Before discussing Nessa's political career, it's important to note her relationship with Frex and the massive impact that had on her character. Nessa is very obviously Frex's favorite, and that favoritism molds her character as well as Elphaba's. In a way, Nessa further ensures Frex's continued favoritism by being exactly what he wants her to be. We wonder what Elphaba's views on religion would have been if Frex had showed her a bit of the favoritism he showed his younger daughter.
So we've gotten Nessa's religious extremism covered, as well as the defining impact her physical deformity has had on her life and her influential relationship with Frex. So how does all this add up to a very powerful and influential political leader? One word: shoes.
Nessa's shoes were initially a gift from Frex and were later magically enhanced by Glinda. The shoes pretty much symbolize her character:
The surface of the shoes seemed to pulse with hundreds of reflections and refractions. In the firelight, it was like looking at boiling corpuscles of blood under a magnifying glass. (220.127.116.11)
The detail about "blood" here implies that there's something almost sinister about the shoes, which is true of Nessa as well. She appears shiny and sparkly and relatively harmless, but she has a capacity for malice that really comes out in her era as the leader of Munchkinland.
"I might bewitch his axe and let it slip," said Nessarose thoughtfully, "just enough perhaps to cut off his arm. I know from experience that a person without an arm isn't as desirable to the opposite sex as one fully armed." (18.104.22.168)
The shoes seem to constantly come between the two sisters. Frex's gift helps transform the adult Nessa into a ruler, but it also obsesses Elphaba and practically dominates the last years of her life. Nessa and Elphaba have a relationship largely based on guilt, blame, and festering resentment.
It's important that Nessa is so largely characterized by her shoes, and that those shoes are what Elphaba desperately longs for. Elphaba basically wants to be Nessa, to have "meaning" the way Nessa does. In terms of Wicked the novel, however, Nessa is actually largely around to provide additional explanation of Elphaba's character.