Demon Snow allows its wielder to materialize a female sword-wielding phantom known as Demon Snow, infamously deemed as the incarnation of slaughter. In Kyōka's case, it initially followed orders coming from a mobile phone she always carries on her person, which was given to her by mother. As such, Demon Snow listened to orders from other people on the line mostly for killing others, and it did not comply to Kyōka's own wishes of not killing anyone anymore. This condition was heavily exploited during her time in the Port Mafia, which earned her position as a mafia assassin. As a result, Kyōka executed assassination missions amounting to a total of 35 kills within six months.
After she joins the Armed Detective Agency, this restriction was subverted with Fukuzawa Yukichi's ability, which enables Kyōka to control her own ability and use it normally without having to rely on her phone as medium. This allowed her to activate Demon Snow and cut off her restraints with its katana while imprisoned in an unmanned aircraft. Though she harbored initial feelings of indifference towards the ability that killed her parents, Kyōka soon came to acknowledge it as the embodiment of her parents' love after learning that Demon Snow protected her life as ordered by them. Being able to freely command Demon Snow to a considerable extent, Kyōka can order it to launch attacks or defend herself whenever necessary, as well as control its manifestation and dissipation at will.
Demon Snow has a high offensive force, capable of lacerating and impaling targets with its katana at quick speeds and successions. In general, it is able to slash through materials and surfaces with relative ease. Nonetheless, Kyōka has shown to control Demon Snow's magnitude or strength of attacks over time. For instance, Kyōka is able to control Demon Snow into merely knocking targets unconscious, which she once remarked to be quite challenging as compared to easily and quickly killing them. Despite its strong offense, Demon Snow is not entirely impenetrable to other abilities, such as Atsushi Nakajima's partially-transformed arm, which was able to block Demon Snow's katana attack. Kyōka was also once able to pin down Demon Snow with household materials, including kitchen knives, a pair of scissors, and chopsticks, rendering it incapacitated for some time and to struggle getting out of its situation.
Aside from its proficiency in handling a katana for offensive purposes, Demon Snow can also perform some basic human movements, such as but are not limited to holding a soap, a dipping bowl, and a mobile phone; shutting a door; and sitting down. Demon Snow is also occasionally used for defense, such as shielding itself and Kyōka from various materials like bullets by slashing through each morsel with its katana.
Demon Snow was once possessed by Kyōka's mother, an ability which she used in various operations together with her husband as professional assassins. Three years ago, assassins being controlled by an unknown assailant who can manipulate bodies through blood infiltrated their residence. Though the assassins were easily fended off by Kyōka's parents, they were eventually controlled by the blood ability as well, urging Kyōka's mother to have Demon Snow slay her husband. She later ordered Demon Snow to protect her daughter, resulting in the ability killing her in the process. In her final moments, Kyōka's mother transferred Demon Snow to Kyōka. However, due to the suddenness at the time, control over Demon Snow was transferred to Kyōka's phone instead to Kyōka herself.
Demon Snow is a reference to Demon Pond (夜叉ヶ池, Yashagaike?), a 1913 play written by the real-life Kyōka Izumi. It may also be a nod to one of the play's characters, Princess Yuki (白雪姫, Shirayuki-hime?, literally "Princess Snow White"), the mistress of Demon Pond and the embodiment of Ryūjin. Forming a portmanteau out of their initial kanji results to "夜叉白雪", which translates to "Demon Snow". The play tells the tale of Shirayuki, a young woman who was offered to Demon Pond as a sacrifice during a drought several years ago. As she became the princess of the pond, she was warned that she could not leave Demon Pond as long as the temple bells toll, else the village would be submerged in floodwater.