Ikari Shinji is one of the most well-developed characters in anime if not all of fiction. Part of this is because his personality was modeled after Neon Genesis Evangelion director Hideaki Anno, who at the time had gone through a long bout of depression. Shinji's personality was further fleshed out using psychoanalytical theories like the Freudian psychosexual model. This combination of personal feelings and actual psychology makes Shinji far more real and nuanced than most anime characters have any need to be.
With how fleshed out Shinji is as a person, it seems like his zodiac sign Gemini should prove itself a perfect match for him. However, there are a number of discrepancies to be found. For example, where Gemini is an active and outgoing sign, Shinji is noticeably introverted and has great difficulty interacting with others. This contradiction between Shinji's personality and his zodiac sign might have something to do with all of his traumatic experiences; had he gotten to live a more normal life, then perhaps Gemini would fit him better. Deciding whether Gemini is truly Shinji's zodiac sign will require a closer look at what the sign means and how it either relates or clashes with Shinji's personality, choices, and actions throughout NGE.
One key factor regarding Gemini is that it always feels incomplete. They feel like they're missing a part of themselves. In an effort to fill that hole, they seek out friends, mentors, and people to talk to. At a glance, Gemini's need for the company of others doesn't seem to fit Shinji. He suffers from something called the hedgehog's dilemma; he wants to get closer to people, but he also fears hurting them, so he instead keeps his distance. This ties back into his low self-esteem and how he seeks purpose for his existence.
Shinji can be said to represent Gemini in his need for the approval of others. As a 14-year-old boy, he's especially vulnerable and insecure when it comes to his self-image. He relies on the approval of the adult NERV staff. It's one of the main reasons he pilots the Eva come whatever trauma. Shinji's need for approval holds especially true with regards to his father. Being abandoned by him as a child is why he questions the value of his life in the first place. He may go through unimaginable pain and nearly die when he goes up against the Angels, but as long as he can hear a "good work" out of his father, it's worth it.
Another part of Shinji's life that either does or doesn't relate to Shinji's zodiac sign is his SDAT. On one hand, he uses it to shut himself out from the rest of the world; this wildly contradicts Gemini's curiosity and fascination with their surroundings. On the other hand, he uses it as a way to feel closer to his father, which ties back into his desire for connections. Gemini is also well-known for its love of music among other art forms.
Despite his self-isolation, Shinji does manage to fulfill the friendship requirements of his zodiac sign. Gemini has a lot of social contacts and loves communicating with strong-willed people. Shinji doesn't exactly "love communicating," but he does have a lot of friends and acquaintances with big personalities, including Asuka Langley and Misato Katsuragi. They also make a lot of social contacts in the office, which, in Shinji's case, would be NERV Headquarters. Anyone seeking evidence that Shinji has a lot of friends should look no further than the final scene of the anime where they're all congratulating him.
Another way Shinji represents this sociability is through his response to Human Instrumentality. If allowed to succeed, Instrumentality would have forced humans to evolve past their physical bodies into a transcendental being with a collective consciousness. While Gemini is an Air sign and not beholden to physical needs, Shinji rejects Human Instrumentality. He recognizes the isolation turning humanity into a singular being would bring and instead chooses to live with and accept other human beings.
For all the ways Shinji can represent Gemini, there are still a lot of ways in which he doesn't. Beyond sociability, Gemini is supposed to be constantly seeking new experiences and intellectual stimulation. They dislike being alone, being confined, and sticking to routines. It's hard to say if any of these traits describe Shinji as he's often quiet and reserved. Also, there are a lot of reused shots throughout NGE, which can definitely be interpreted as routine.
If anyone wants to call Shinji a Gemini, they'll have to carefully observe his actions for answers. He's an unbelievably nuanced character, so there are plenty of ways he can either perfectly embody or greatly contrast his zodiac sign. Humans are complicated, and Shinji is as human as anime characters come.
