Back before the Evangelion Rebuild series, the most complicated character in Neon Genesis Evangelion (besides basically everyone) was Kaworu Nagisa. However, the rebuilds brought with them a new character that would be loved and hated in all the right ways that only the Evangelion series could conjure, and audiences still aren't sure what her deal is.
Mari Makinami Illustrious first appeared in the opening scene of Evangelion 2.22 You Can (NOT) Advance, where she piloted the Eva Unit 5 before its destruction. From then on, she appeared throughout the second film before becoming a mainstay in the third film onward but remained such a mystery as to be an annoyance to some. There is a lot to be said about Mari, but far more to be theorized, and so much of it conflicts with each other, from her significance in the plot to her thematic importance. Her existence could even be emblematic of the chaotic production of these films, but a lot of what is thought about this character is both oversimplified and overcomplicated.
Let There Be A Glasses Girl
Mari was constructed entirely out of a request of a producer to make a new female character in a cast known for creating iconic anime women. Reading through the development of this character, it almost seems as if her existence was a hindrance on the story, with accounts of script revisions and an uncertainty of who she actually was.
Before her name was even revealed, the fandom had already taken to calling her "Glasses Girl" and were excited to learn more about her. Little did they know that Mari's construction was so chaotic that not even the directors and writers truly knew. Anno's assistant director and the man behind FLCL and Diebuster, Kazuya Tsurumaki, took it upon himself to shoulder her creation.
Tsurumaki and FLCL's writer Youji Enokido worked together to create the character of Mari, but it was hard to make a character that didn't echo other existing female characters. Attempts to write her as a rival to other characters were met with skepticism, not wanting Mari to step on characters like Asuka.
The broad ideas that went into Mari's construction left Tsurumaki believing that this character was one with "multiple personalities," something fans of the series can certainly see in her execution. Each scene sees her switch modes, sometimes vicious and angry and other times quirky and happy.
How Mari "Destroyed Eva"
To say that Mari "destroyed Eva" sounds like a condemnation of her character entirely, which for many fans was probably the feeling, especially in the time after 3.33. She showed up a few times in 2.22, mostly in big action scenes, and never really was explained. Then, in 3.33, she was a principal character, but the film was focused on Shinji, so Mari once again was a mystery.
In the long wait between the third film and 3.0+1.0, many fans who were unhappy with the Rebuilds up to that point were very critical of Mari as a character, seeing her as pointless. Considering the tumultuous process of turning her into a character, a lot of that behind-the-scenes insight validates those criticisms.
However, destroying Eva isn't so much a consequence of her character, but rather the goal of her character, much the same as the mission statement of Eva 2.22. In Japan, the first three films were given the subtitles Jo, Ha, and Kyu respectively, named after the acts of Japanese theater. 2.22's title, "Ha," means "Break."
"Break" can be interpreted to mean breaking away from the established Evangelion story, but it can also mean that it is breaking Evangelion so that it can truly be rebuilt as the film intended. Mari appears as something unknown and undefinable and ended up being one of the most memorable parts of the second film.
Mari is Fun
Everything about Mari in 2.22 makes the film so fun. Her first fight scene is exhilarating and then the next the audience sees her, she falls from the sky in a parachute right onto Shinji. The arrival of this alien figure is the beginning of something completely new for Evangelion after the first film was essentially a recap of the first six episodes of the TV show.
"Alien" is an apropos way of describing her, as that's exactly how actress Maaya Sakamoto prepared to take on a character that was so undefined in concept. The result was a character that audiences either loved or hated, and even the English performance by Trina Nishimura captured this unwieldy appeal with lots of enthusiasm and some flavorful swearing.
Mari wants nothing more than to be an Eva pilot as a form of thrill-seeking, whereas Rei does it to fulfill her purpose, Asuka does it to prove herself, and Shinji does it despite not wanting to. She loves to sing, she loves to read, and she seems to be the only character that loves to be alive. And sad as that last detail is, that makes her truly alien in this world.
But Who is She Really?
Spoilers for Evangelion 3.0+1.0 ahead.
In Evangelion 3.0+1.0, there are a lot of questions finally answered and during one of Gendo's speeches, the audience is shown imagery of Gendo during his youth in college. This is where he met Fuyutsuki and Yuki, Shinji's mom. But Mari - and presumably Asuka's mom - were also there.
It is heavily implied in this case that Mari became eternally young through the "curse of the Evas" the same way Asuka did and that she's been alive for about over 40 years. It isn't certain what she did precisely in all that time, however. Towards the end, Fuyutsuki refers to her as "Maria Iscariot," to which she replies that she hasn't heard that name in a while.
The significance of that name is unclear, whether it's just more religious undertones, whether it implies Mari is something more than human, or if it's a pun based on her betrayal of Gendo. Like with a lot of Evangelion, there are numerous ways to look at it.
In the end, Mari Makinami was a character that wasn't supposed to exist, that arguably only exists for commercial reasons, and yet still managed to be thoroughly entertaining. She was simply a hit for completely different reasons than any other girl from Evangelion before her. An alien both to the production and the story, yet somehow... she just works.