Understander

Understander (理解者) is a recurring term that primarily used to describe the relationship between Kamijou Touma and Othinus. First appearing in Shinyaku Toaru Majutsu no Index Volume 13, it is later used to describe the relationship between other characters in later novels.

Principles
After Touma experiences countless worlds due to his attempt to defeat Othinus, they later find themselves in a unique position of being the only ones to truly understand each other due to their shared experiences. From then on, the two share a closeness and bond with one another unique to each other, the term "Understander" first being used by Othinus toward Touma.

Based on how Kamijou Touma and Othinus interact with one another and how the term is used, the term "" is the closest thing that can describe their type of relationship. Though romantic and sexual feelings are not excluded from the relationship, as shown with the various times Touma and Othinus "flirted" with one another, the relationship transcends that. According to Othinus, she will always be by his side no matter how much ugliness Touma revealed, then proceeding to call him a piece of garbage. According to Ollerus after Silvia's brutal beatdown of Kamijou Touma, him dying would turn her into an even more terrifying monster. Meanwhile, he says that if she died there, a great change might come over his nature. Moreover, when Othinus "died" in Touma's arms after their battle in Denmark near Egeskov Castle, the Magic Gods became so concerned of its effects on Touma and the Imagine Breaker that they intervened and turned Othinus into a "fairy" instead.

These indicate how deep their relationship is to the point of even changing their natures if one of them died. Indeed, even Touma notes and reiterates how much Othinus understands him after she points out why he doesn't need weapons to save people. It was her words that later reminded him not to use a weapon (UL Exploder) against Kihara Yuiitsu when she attacked him and Salome, much to his detriment.

In spite of this understanding and trust towards one another, misunderstandings and new revelations still prop up for both of them. A minor one would be Othinus being unaware how rare it was for Touma to ask others for help when Touma relied on her for them to escape Sargasso. A major one would be Touma not understanding Othinus' feelings and tried punishing herself for Touma's sake, as pointed out by Thor.

Other Examples
The only other example of this relationship in the series is with the relationship between Samuel Liddell MacGregor Mathers and Mina Mathers, who were also married. The narration itself states that their relationship is similar to that of Touma and Othinus', stating that even if Mathers pushed Mina away or insulted her on the surface she would never mistake the meaning behind his temper.

Despite this, the narration states that Mathers had multiple "Understanders" beyond Mina. This implies that a person can have multiple Understanders.

During Aleister Crowley's battle with the tarot version of Mathers in Scotland, one of the many things he had envied of Mathers was that he had multiple Understanders and him not having a single one. This indicates Aleister wanting an Understander for himself.