Avery was convicted of the murder of photographer Teresa Halbach and will be represented by the best of the best.
Zellner has been practising law for three decades and has righted more wrongful convictions than any private lawyer in the US, according to her firm's website.
As well as uncovering the wrongs in the state's witness testimonies, Zellner is feeding Making A Murderer fans, offering a reading list for those eager to know more.
"Brendon Garrett's book "Convicting the Innocent shows how often judges get it wrong by rubber stamping prosecutions. #MakingAMurderer," she tweets.
But while Avery's right-hand woman works to unmake a murderer, "one false conviction at a time" head of the prosecution Ken Kratz has announced he will prove Steven Avery's guilt in his new book.
In a letter tweeted by Zellner, Kratz calls on Avery to come forward and "tell the truth" in his book.
"Since I'm the person who probably knows more about your case than anyone else, I hoped that you would choose me to tell your story to," Kratz writes.
"Unfortunately, you only want to continue your nonsense about being set up."
"That's too bad."
"If you change your mind and want to tell your honest story someday, please contact me."
Don't think for a minute Zellner let this slide, the appeal lawyer lashed out at Kratz tweeting: "Kratz letter to SA: Confess so I can write book & profit. This bloodsucking gives vampires a bad name #MakingAMurderer."
- AAP