|Spoiler Alert: This article/section contains details about future plot lines described in the light novels, not covered in the anime. Do not proceed unless you want to be spoiled.|
Inverse Form (反転形, Hanten-tai?) occurs when Spirits are exposed to severe negative emotions. In this transformation, they undergo a radical change in appearance and emotions, their Reiryoku energy readings to go straight down into negative parameters at high levels, and gain an armament called a Demon King in the place of their Angel.
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. Inverse Spirits have a distinct "negative" Reiryoku that Ratatoskr classifies with a Category E alarm.
Appearance-wise, Spirits gain an altered dress. When Tohka 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. Their Angels are also replaced by Demon Kings, which 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. Inverse Spirits are generally shown to be mindlessly destructive with no ability to distinguish friend from foe, but in the case of Tohka, another personality will take control of her body.
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 Mukuro Family, Inverse Tohka asserts that Qlipha Crystals are the natural form of Sephira Crystals, making Inverse Spirits the default state, but they were changed to their current condition to make them more compatible with humans.
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. Both Westcott and Kotori displayed knowledge about the Inverse Form just as Tohka entered it the first time, suggesting that there has been extensive research done about the transformation.
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. Reine noted while observing the transformation that Tohka's spirit readings went down to zero and then continued to go down into the negatives. While Inversed, even Ellen, the strongest Wizard in the world, wasn't even able to harm her. Although Ellen did imply that she could fight Tohka were she not already wounded. 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.
Tohka would Inverse a second time when Mukuro sealed away all memory of Shido, accumulating stress from the subconscious feeling of losing the most important person in her life. Mukuro herself would later start to Inverse after unintentionally grievously injuring Shido and recovering her memories of her adoptive family rejecting her.
In the past, Kurumi nearly Inversed after realizing that the monsters Mio had tricked her into killing were in fact former humans giving Qlipha Crystals, and had just killed her best friend Sawa. However, she used her Fourth Bullet <Dalet> to rewind herself to a state before the despair could set in. The White Queen, who has all the powers of Kurumi's Inverse Form, was born as a temporal clone reproducing the moment of her near inversion. While initially incomplete, the clone would merge with Sawa Yamauchi in the Neighboring World, giving birth to the current form of the White Queen.
In one timeline, Ellen successfully kills Shido in front of Tohka, Origami and the Yamai sisters, causing all four Spirits to Inverse from the despair of losing their beloved. However, Kurumi would obtain her Sixth Bullet <Vav> that she had unwittingly sealed from Shido's corpse and sent her consciousness back in time to avoid this scenario.
- 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.
- Inverse Spirits are designated with an imaginary number "i" next to their Spirit number on the light novel covers.
- Ellen claimed that if she were to rank the strength of Tohka's Inverse Form, it would be AAA rank. However, this is the same rank as Tohka's Spirit form and Ratatoskr's data about the Spirits ranks Tohka in her Inverse Form to be SS rank.
- The author explained that Inverse Spirits gain more revealing Astral Dresses because he associates evil female characters with seductiveness. 
- Despite having not made an appearance in the main story, an extra illustration detailing Yoshino's appearance in her Inverse Form was made for Date A Live Tsunako Art Book SPIRIT.