Warning: Major spoilers for Kara no Kyoukai ahead.
Normal and abnormal. Life and death. Male and female. Across its eight movies and two OVAs, Kara no Kyoukai (AKA The Garden of Sinners) looks at all these ideas and more. The world of Kara no Kyoukai is full of opposites like these, opposites that none the less balance each other and rely on each other to exist.
This duality is apparent in Shiki’s character from early on. When Kokutou first meets her, Shiki has two personalities, one male (SHIKI) and one female (Shiki). They share memories, but are complete opposites in every other way. Shiki is quiet and reserved, while SHIKI is blunt and outgoing. The second movie is characterized by Kokutou trying to grow closer to Shiki, but never fully connecting with her. They’re drawn to each other, even when neither one completely understands why. Shiki continually attempts to push him away by telling him about her desire to murder, but that doesn’t diminish Kokutou’s desire to be with her. At this point, Shiki Ryougi is a complete person. She has a male half and a female half that have achieved a kind of balance, a balance that becomes unstable with the introduction of Kokutou into her life. Kokutou is the polar opposite of Shiki: he’s an abnormally normal person with a firm conviction that murder is wrong, while she’s decidedly abnormal and always has an urge to murder someone. Shiki and Kokutou are drawn together because they’re such different people, but can’t fully connect because SHIKI and Shiki are already a complete whole, and Kokutou’s influence starts to disrupt that balance. As Shiki’s internal balance collapses, her feelings toward Kokutou eventually explode, leading her to try to kill him, and then choosing to try to kill herself rather than live near the normalcy she could never have. Instead, SHIKI dies, leaving Shiki alone.
After SHIKI’s death, Touko comments several times that Shiki has an emptiness inside her, an emptiness caused by losing SHIKI. Without something or someone to balance her out and fill the emptiness SHIKI left behind, Shiki is incomplete. As she lies in the hospital, she struggles to even feel alive. It’s here where she and Kokutou first start to connect. It’s little things at first, like Shiki letting Kokutou spend time at her apartment and trying the ice cream he bought her even though she didn’t like it, but their relationship soon grows deeper. Kokutou is the first person to make her smile after SHIKI’s death, and he later promises to carry Shiki’s sin in her place. Shiki and Kokutou may be opposites, but they also complete each other.
The fifth movie, Paradox Spiral, explores this duality even further. When Tomoe Enjou meets Shiki shortly after killing his parents, the two hit it off quickly. Shiki lets him move in with her, and Tomoe starts to fall in love with her as the days go by. In spite of that, there’s always something off about their relationship; they don’t have the same magnetism as Shiki and Kokutou do. They spend much of their time just lying around the apartment, going about the same routine every day, and doing very little in general. When the two of them are together, they only stagnate.
Being a murderer himself, Tomoe can’t complete Shiki the way Kokutou does. Tomoe is already abnormal in the same way Shiki is, which keeps him and Shiki from ever connecting and completing each other the way Shiki and Kokutou do. Shiki’s dream at the end of the film after Tomoe’s death only serves to drive this home. The two of them are sitting in mirror images of the same restaurant, and slowly get up and walk away from each other. They might be reflections of each other, but reflections can never truly touch. In spite of, or rather because of, their similarities, Shiki and Tomoe can mirror each other, but never connect. Completeness only comes from two opposites, not two reflections.
The power in this concept is made clear in Araya’s master plan. His goal is to reach the Root, the place where all the knowledge of the world is stored, by using the power inherent in yin-yang. His goal is to fuse with Shiki, combining his yang and her yin, and use her Mystic Eyes of Death Perception and the spiral apartment building, also symbolizing yin-yang, to ultimately reach the Root, an embodiment of perfection. Yin-yang itself represents two opposites in perfect balance, and it’s only through the combination of these two opposing forces that Araya can finally reach perfection.
By movie seven, Shiki has reached a kind of balance again, in large part thanks to her relationship with Kokutou. The biggest threat to that is Shirazumi, someone solidly on the abnormal side of the world. He’s a serial killer who’s been awakened to his Origin through supernatural means and seeks someone like him. He targets Shiki because the old Shiki was just as murderous as him, but her relationship with Kokutou provides an anchor to the side of normalcy. As much as her instincts tell her to, Shiki can’t bring herself to kill Shirazumi because of Kokutou. Their relationship leaves Shiki precisely on the boundary between normal and abnormal. Shirazumi constantly attempts to drag her to the abnormal side, but Shiki only commits and kills him when she believes Kokutou is dead. Once she discovers that Kokutou is actually alive, Shiki has her anchor once again. Even though he said he would never forgive her if she killed someone, Kokutou still accepts Shiki, murderer and all. Her abnormality and his normalcy only reach perfection when they’re together. The two of them represent the perfect balance of opposing forces.