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Inverse Forms (反転形 Hanten Katachi?) occurs when a Spirit is exposed to severe negative emotions. In this transformation, they undergo a radical change in appearance and emotions. This form also seems to cause a Spirit's Mana readings to go straight down into negative parameters.
In this state, the Spirit in question gains an armament called a Demon King in the place of their Angel. These Demon Kings appear to be darker equivalents of said Angels, and are either much more powerful or simply unrestrained by their user's new lack of moral limitations.
According to Westcott, this is the Spirit's true form in the bordering dimension, as the form they arrive in the main dimension is their normal form. This could also bring some insight into Tohka's description of what it's like to enter that dimension and return, as arriving there causes her to apparently "go to sleep" while coming back feels like she's being forced to wake up.
Appearance-wise, Spirits gain either a slightly altered Astral Dress or a heavily altered dress. When Tohka became Inversed, her outfit turned darker and more revealing. When Origami inversed her astral dress also turned darker and more revealing, but parts of her outfit were changed as well, like the acquirement of a longer veil and different skirt.
In Itsuka Disaster, Westcott refers to Origami and Tohka's Inverse Forms as a "perfect inverse type" but it's unknown what he means by this as of now. He also mentions that a Spirit going into the Inverse Form is rare, hinting that its very hard to make a Spirit inverse without putting them into what he describes as an "abyss of despair".
In Nia Creation, it's revealed the Inverse Forms aren't powered by a Spirit's Sephira Crystals, but by their counterparts: the Qlipha Crystals. Nia had her Qlipha Crystal removed by Artemisia after assuming her Inverse Form. In the process, Westcott absorbs the crystal and steals Nia's ability to use <Beelzebub>.
All known cases of a Spirit going Inverse were the result of them feeling intense negative emotions such as despair, usually as someone close to them is about to die or immediately after they have died.
The first known case of a Spirit going Inverse was with Tohka, the trigger for this case was when she was filled with intense negative emotions when Ellen was about to kill Shido right in front of her eyes. While Inversed, even Ellen; the strongest Wizard in the world wasn't even able to harm her, while Ellen did imply that she could fight Tohka were she not already wounded, she and Westcott chose to retreat. While in this form, Tohka has proven to have no memories of her normal self, and has shown no hesitation to kill anyone, including Shido, in fact she ironically seems to register Shido as a threat and believes he is trying to deceive her because of the normal Tohka's residual memories of him.
Seemingly for no other reason than to kill Shido, she destroyed a massive portion of the city with her most powerful sword: Paverschlev, despite clearly being already more powerful than everything around her without it. Also, after returning to normal, she had no memories of what happened while in this form, but Tohka retains at least an instinctive feeling that nothing good will come from this power if she ever uses it again. Dark Tohka seems to at least retain knowledge about her normal self's Angel Sandalphon, as she easily recognized it when Shido wielded it.
The second known case of a Spirit going Inverse happened to Origami soon after she was transformed into a Spirit by Phantom. The trigger this time was caused when she went back in time hoping to save her parents only to end up accidentally killing them herself with a rogue blast and effectively realizing that the Spirit who killed her parents and caused all of her suffering was none other than herself. In her case, while in this form, she has no control over her body at all, attacking everything and everyone within her sight randomly. She appears completely unresponsive and incoherent, as opposed to Tohka, with Shido even noting that she looks like she's given up all hope based on the dead look in her eyes.
The third known case of a Spirit going Inverse happened to Nia, as Westcott restored her repressed memories from all the torture she endured while she was captured by DEM. However, unlike the previous cases, her Qlipha Crystal was removed by Artemisia through an unknown method. As a result, she reverted back into her normal state, seemingly losing the ability to enter her Inverse Form.
- Ellen claimed that if she were to rank the strength of Tohka's Inverse form it would be AAA rank. This is the same rank as Tohka's Spirit form.
- However, Tohka's actual rank while in her Inverse Form is confirmed to be SS rank in the Ratatoskr's data about the Spirits.
- It can be inferred that Ellen, since she would've already known "Princess's" rank, was in fact commenting that Tohka's state in inverse was worthy of being given it, unlike her sealed state beforehand which clearly had inferior power.
- Both Westcott and Kotori displayed knowledge about the Inverse Form just as Tohka entered it the first time, suggesting that other Spirits have transformed into this state before Tohka.
- It is currently unknown if Shido can seal the power of an Inverse Spirit as Tohka only returned to normal when Shido's kiss let her remember who she was.
- In this same vain, Tohka's Spirit powers were still sealed inside of Shido because he was able to call Sandalphon when she transformed and even afterwards yet her Inverse form's power was easily at its fullest. Likewise, he needed to bring both Origami and Nia out of their inverse states before sealing them.
- Reine noted while observing the transformation that Tohka's spirit readings went down to zero and then continued to go down into the negatives. This likely means a spirit can't transform into its Inverse Form if their readings have not reached a negative level.
- Inverse means opposite or contrary in position, direction, order or effect. This is likely why the Demon Kings a Spirit uses in their Inverse form are named after the Demons of the Qliphoth, who are essentially believed to represent the evil counterparts to the Sephirot's Angels.