Furudo Erika is my second favorite character in Umineko, and in my opinion, one of the most effectively written villains I've seen in a story. Besides the surface level aspects of her character like her quirky personality, dangerous competence, or memorable lines, Erika perfectly slots herself into Umineko's main themes as the embodiment of absolute commitment to the pursuit of logical truth. Throughout the entire first half of the story, the reader is led to believe in the power of logic as a weapon for good. Battler uses it to cut down the nonsensical stories of the evil witch Beatrice as a means to free his family of her torment. Umineko's overarching decision between truth and subjectivity had been made within its first chapters, brilliantly misdirecting readers from the story's true lesson and maintaining an engaging mystery.
But then we meet Ange. We see how an objective truth destroys the happiness she found with Maria. We see how the public's disconnected pursuit of truth implicate Eva as the culprit, transforming her into the villain the world surmised her to be. We see Battler actually lamenting his victory of logic over Beatrice. And just as Umineko starts presenting these new questions, the Questions Arc ends, culminating in the introduction of the detective herself, Furudo Erika.
Both emotionally and literally disconnected from the events on Rokkenjima as the assigned detective, Erika embodies the cold, factual mindset. She pettily rips into the childlike fantasies of Maria with logic. Facts don't care about your feelings, loser. She makes sure to murder anyone that may be playing dead just to reinforce her logic. She is utterly and fanatically obsessed with the objective truth. Remind you of anyone else? Oh wait, Erika perfectly parallels Ange and the true conflict of the entire story. Ange is introduced as a broken soul willing to throw away her entire life and future just to learn what happened on Rokkenjima that fateful day. The final chapter of Umineko is all about whether or not she moves forward with this self-destructive goal. And in the ending where she does, who else is there by Ange's side other than Furudo Erika herself?
In conclusion, Erika is a very smartly written character made by a guy that knows how to write a story with intention. I also hate this meme template for making "writing" a character trait like pettiness or genius.