Resanerié Sadis Dialine

Resanerié Sadis Dialine (レザネリエ＝サディス＝ダイヤライン) is a third-year student of Tokiwadai Middle, the school's debate representative and head of its equestrian combat club, as well as the young Empress of the Dialine Empire in Eastern Europe.

Appearance
Resanerié has shoulder-length red hair, with jewelled tiara worn on top. As a student of Tokiwadai, she wears the school's uniform, with thick leather riding breeches on her legs.

Background
During her time in Academy City, she acquired the loyalties and services of four ordinary girls, separate from her retainers back in the Dialine Empire, who would be the debate representatives of the other four schools in Garden, with Resanerié being Tokiwadai's debate representative. Tougyo Kea served as her attorney, Renka Ephilty as her maid, Sanki Ai as her accountant and Shitatsuzumi Yoshiko as her chef.

At some point in the past, Resanerié's beloved horse Bistorio (ビストリオ) broke his leg in a training accident and was forced to retire. Considering that his treatment costs and genetic value were not worth it, without Resanerié's consent and against her wishes, the adults removed his medical equipment and denied him further medical care or euthanasia, letting him die in pain. Besides her rage and sorrow at having lost her partner, Resanerié realized that the adults did not treat humans any better, and the same could happen to any of the students, with the additional possibility of something happening to her to preserve the secrets of esper development when she was due to return to her own country. She subsequently devised a plot to take Garden to the skies away from Academy City and have everyone in it declare independence, which she saw as the only way to protect themselves from any such fate, however she hid her own personal reason, perhaps believing that it wouldn't be a good enough reason for someone to fight and giving up on the idea of anyone helping without actually asking honestly.

For some time, Resanerié utilized the resources at her disposal to secretly prepare the underground foundation necessary for her plot, including the propulsion and reinforcement necessary to make Garden airborne and factories to supply the population with food. At same time, she and her servants would plan and lay the psychological groundwork in their positions as debate representatives.

Toaru Kagaku no Railgun (light novel)
On the day of the entrance exams, Resanerié executed her plot to take Garden into the skies and make it independent from Academy City under her control. Shortly beforehand, Yugure Kanaria discovered the underground infrastructure, leading Resanerié's group to attempt to eliminate her by weakening her from a distance and shutting the exit to trap and starve her, given that she couldn't reach the factory's products from her location and they could not approach her due to her deadly Micro Dying ability. Afterwards, Resanerié and her allies conducted a live debate in their positions as debate representatives, sowing their ideas among the populace prior to take-off.

At some point after Garden became airborne and they took control, Resanerié was agitating a crowd of students into supporting the execution of teacher Akazame, and when Misaka Mikoto intervened, she manipulated the crowd against her. The two briefly clashed on the school roof, ending with Resanerié using the boost to her halberd from her ability to overwhelm an unaware Mikoto's defences and knock her from the roof. As the Level 5 retreated, Resanerié was greeted by her retinue as her weapon crumbled and she prepared to send several students (including Shokuhou Misaki's clique, who were forced to feign allegiance for the time being) to flush Mikoto out as a loyalty test.

After taking a bath and recalling her former steed, Resanerié received a new halberd and learned that Mikoto was heading for Tokiwadai while fighting with the Shokuhou Clique, with the two putting on the act of a battle while covering their movements and exchanging information. Resanerié was waiting for Mikoto in the school courtyard and the two began their rematch, with Resanerié going at full power to end it fast so she could join her retinue in silencing Yugure Kanaria and Uiharu Kazari before they could expose her plan's weaknesses. Now aware of Resanerié's power, Mikoto was able to counteract her enhanced movements, and although Resanerié was able to weaken her Railgun and iron sand sword, she was able to adapt accordingly.

As Resanerié brought up the fact that as Empress she would eventually have to return to her country and implying that the measures that would be taken then due to her esper power were why she acted, Mikoto deduced that wasn't the root cause and noted Resanerié's lack of a horse, bringing the true reason behind Resanerié's actions to light and shaking her. As Mikoto expressed the thought that the original reason of revenge for a life being unjustly taken was cooler and more becoming of her than the independence nonsense that had been presented, Resanerié resumed her attack, but Mikoto was able to snare her halberd with her blazer and finished the battle with a close-range railgun shot.

Once Garden was returned to the ground, Misaka Mikoto and Shokuhou Misaki used their respective abilities to cover up the incident as a surprise psychological experiment and technological demonstration, but Shokuhou left Resanerié's memories (including those of her horse, the instigating factor) intact.

Abilities
Resanerié's ability is Level 4 Crimson Manager, which allows her to manipulate rust. She is capable of rusting a rustable target within her proximity, and can also telekinetically control rust to move rustable objects with considerable force and speed, accelerating the rusting of those objects in the process.

Resanerié is skilled in the use of the, both on foot and on horseback, wielding it one-handed (her other hand for the reins). She can supplement its destructive power and speed with her esper ability,  but at the risk dislocating her shoulder without stretching first to loosen up the joint,  and at the cost of her halberd rusting and eventually crumbling away (even with resistant stainless steel and titanium alloy, the latter estimated to not last 10 minutes).

Being the captain of the equestrian combat club, she is presumably skilled at riding a horse in addition to horseback combat, though she has lost her partner Bistorio. She is also skilled at debating and psychological manipulation, illustrated in her position as Tokiwadai's debate representative and her manipulation of Garden's students during her plot. With the latter, she also wielded a megaphone that besides producing sound also spread an itching chemical to further agitate her targets.

Trivia

 * According to Kamachi Kazuma, Resanerié came from that fact that it had been said that Tokiwadai would turn away a real princess if they didn't have what it took at the entrance exams, and the fact that a truly royal Tokiwadai student hadn't been seen yet, as well as him not having done a girl in horse riding clothes in the story yet.