Why audiences can't take their eyes off Making a Murderer's Kathleen Zellner.

Very rarely, you come across a person who is equal parts fiercely intelligent, unwaveringly determined, deeply passionate and utterly terrifying.

Attorney Kathleen Zellner, who has righted more wrongful prosecutions than any other private attorney in America, is one of those people.

In Making a Murderer season two, Zellner features as Steven Avery’s lawyer, who is working pro bono to uncover Avery’s innocence in the 2005 murder of Teresa Halbach – for which he is serving life in prison.

Throughout the season, she meticulously works through the evidence in the Avery case and tests it, to find hole after hole in the story built by the State about how Avery murdered the 25-year-old photographer.

From the blood found in Halbach’s discarded RAV4, to the bullet that was said to have killed her, Zellner is able to raise complex and important questions about the case. For her, there are facts that just don’t add up, and crucial elements of the crime scene that weren’t considered at Avery’s 2007 trial.

She believes, and relentlessly argues, the evidence that ultimately resulted in Steven Avery’s conviction was planted. At this stage in the legal process, she is also able to point the finger at other potential suspects, examining potential motives and alternative theories that might explain how Halbach was murdered.

As someone who isn’t a legal expert, I don’t have a strong opinion about whether or not Steven Avery is guilty. I understand that the Netflix documentary, written and directed by Laura Ricciardi and Moira Demos, is about telling a compelling story and crafting a narrative for an audience to become invested in. I’m certain details have been omitted, nuances have been brushed over, and complex legal concepts have been simplified for the purposes of entertainment.

What I do have a strong opinion about, is Kathleen Zellner.

Watch the trailer for season two…

The 61-year-old is striking in her sense of self-assurance. Despite what she’s up against, despite getting knocked back by the legal process, and despite cruel commentary online and from people in the media, her confidence is entirely unwavering. She believes, wholeheartedly, that she can prove Avery is innocent. And if one approach doesn’t work, she simply tries another.

Watching Zellner is like watching a masterclass in problem solving.

She dissects each and every part of the case against Avery, and intricately examines it, artfully placing it back together again to show where the gaps are. Even once she’s submitted a 1,300 page petition about the problems with evidence used to convict Avery, she keeps going. She adds to it, again and again, as she unearths more details no one else has come across.

A notable part of Zellner’s work ethic is her grass roots approach. She asks her staff to look into certain aspects of the case, but her motto – ‘trust no one’ – means that she does everything herself. She reads all the documents, she looks at the evidence, and she goes to the crime scene. That’s how she notices the tiny mistakes made during the 2007 trial that could – if her theory is correct – free an innocent man.

Working with forensic experts, she buys a RAV4, identical to Halbach’s, to test the State’s theories about how the murder unfolded. What starts as small, seemingly far fetched experiments, come together to peel back what we all think we know about Avery’s case.

With every expert Zellner works with, her thirst for knowledge is palpable. She wants to understand how someone goes about analysing a bone fragments, or blood spatter, or a burn site. What questions do they ask? What conclusions can they draw? How certain can they be?

What's clear immediately - and becomes increasingly clear throughout the season - is that Zellner is brilliant at what she does. Since opening her law firm in 1991, Zellner has overturned 19 convictions, including that of one man who was sentenced to the death penalty for a crime he didn't commit. In her work, she's convinced guilty people to confess and key witnesses to recant their testimony, as well as used DNA evidence to exonerate innocent individuals.

Making a Murderer season two paints a picture of just how difficult it is to overturn a conviction. It requires meticulous, relentless and frustrating work, that often takes years and years. It takes resilience and self belief that even when everyone is telling you to give up - you keep going.

These attributes, however, aren't at odds with having a moral compass. In fact, she has a very strong one. After representing serial killer Larry Eyler, who died in 1994, Zellner said she never wanted to defend a guilty person again. In Making a Murderer, she says you'd be stupid to hire her if you were guilty, because she'd find out.

This is someone who believes in what she does and is exceptionally talented at it. As a result, viewers of the series cannot stop talking about her.

"If someone’s innocent," Zellner says during the series, "you find a way".

She's fierce, she's blunt and she's sharp. And she shows us that more often that not, the secret to success isn't to get it right the first time, but to keep trying until you do.