AFAIK, Kisara is a regular human who has no superhuman habilities (correct me if I'm wrong). Why when she was attacked by Tina Sprout she was able to cut Tina's metal gatlin gun in half with her sword? Is because she's a very skilled martial artist? Does she have some superhuman hability or metal prosthesis that makes her stronger like Rentaro? Is her sword a special sword or it's made of varanium or something? What is it?
You are correct, Kisara is in fact just a regular human as pointed out in the passage below
Rentaro looked at the scene dumbfounded, shaking. He considered—Who was the strongest member of the Tendo Civil Security Agency? Was it himself, who possessed the abilities of a mechanized solider? Or Enju, who manifested the superhuman physical powers of the Cursed Children? Or Tina, who had the qualities of both? “Papa, no! She’s the most dangerous out of everyone here!” The night before Operation Rapier Thrust, when Tina and Kohina were about to fight, this is what Kohina shouted about Kisara when Kisara went to intervene. Even though Kohina thought that she could win or at least put up a good fight against Tina and tried to continue their match, the moment Kisara intervened, she put her swords away. Even Kohina, an Initiator, understood that she was at too much of a disadvantage if she had to fight Kisara. Against Kisara, an ordinary human.
Kanzaki, Shiden. Black Bullet, Vol. 4 (light novel): Vengeance Is Mine (p. 204). Yen Press. Kindle Edition.
So while Kisara is just a regular human she is insanely powerful, so powerful in fact that even an Initiator as strong as Kohina does not want to fight her. She has been trained in the Tendo Martial Arts (similar to Rentaro) since a very young age and has even developed several of her own secret techniques in her pursuit of revenge against the Tendos.
She’s beautiful, thought Rentaro from the bottom of his heart as he looked up at his fellow pupil, at the same time unable to suppress a shudder. There was not a single opening in her stance, and there was something about it that made Rentaro certain that the moment he stepped within reach of her blade, he would be cut down. Rentaro discreetly took his smartphone from his pocket and looked at its LCD screen. It was almost time to leave for school. She would move soon. Just as he thought, there was almost no need for him to wait. Kisara exhaled softly, and her clear voice rang out. “Tendo Martial Arts Sword Drawing First Style, Number 1—” Her scabbard rang, and her sword was unsheathed with the speed of a lightning bolt: “Tekisui Seihyou.” The slash made an extremely modest whoosh. But the top half of the target in front of Kisara—a carved wooden target wrapped in cloth—was blown off with a destructive blast, and pieces of it flew into the dojo’s walls. The most astonishing thing of all was that there was a distance of more than six meters between Kisara and the target. Rentaro gulped. The striking distance of a sword catapulted by its sheath to accelerate when drawn was equal to the length of the sword plus the length of the swordsman’s arm and step. However, the Tendo sword-drawing technique has something more than that. It was not like Rentaro had seen all of Kisara’s techniques, but he knew that she could slash up to three times her striking distance. And this was data from three years ago…
Kanzaki, Shiden. Black Bullet, Vol. 2: Against a Perfect Sniper (pp. 13-14). Yen Press. Kindle Edition.
Now, about her sword Yukikage (or Snow Shadow) there isn't much revealed in any of the light novels, the passage below contains about the most information
Then, he realized. Rentaro thought she had been using her usual sword-drawing practice sword, but although the black scabbard and base and red cord looked similar, this was a real sword. “The murderous blade, Yukikage, huh…?” “That’s right.” As Kisara stopped working and looked at him, she dusted off the scabbard and held it up to the sunlight coming in through the window. Bathed in the morning sun, the wavy temper pattern on the blade scattered the light, filled with a charm that drew in all who looked at it. Kisara looked at the blade absentmindedly and murmured, “Satomi, did I ever tell you what ‘murderous blade’ means?” “No…” “In Zen, it’s the opposite of the life-saving sword; it negates all human delusion. This…is the sword that will hunt down all the Tendos, Satomi.”
Kanzaki, Shiden. Black Bullet, Vol. 2: Against a Perfect Sniper (p. 15). Yen Press. Kindle Edition.
During the fight with Tina in the office however, it is explicitly mentioned that Rentaro hears the sound of steel as Kisara slides the blade out of the scabbard
“Kisara! The floor… Cut it………!” He could sense Kisara behind him suddenly leaping to action with the sword in her hand. “Tendo Martial Arts Sword Drawing Third Style, Number 8—” Her deep exhalation made the air in the room tense in a second. With all the noise from the gunfire, he shouldn’t have been able to hear it, but Rentaro was sure he could hear the clear sound of steel sliding out of the scabbard. “Unebiko Yuusei—Go swiftly, Yukikage!” The calamity that occurred immediately afterward could scarcely be called a physical phenomenon. It felt like Rentaro’s field of vision was cut into a number of pieces, and along with the cracking sound of an icicle being cleft in two, all around the room were cuts carved in every direction.
Kanzaki, Shiden. Black Bullet, Vol. 2: Against a Perfect Sniper (p. 108). Yen Press. Kindle Edition.
Given that it is never explicitly mentioned that Yukikage is made from Varanium (in the light novel it is always mentioned when blades/weapons are Varanium like Kohina's swords) and given its coloring and the mention of steel in the passage above we can probably assume that it is just a regular Samurai sword (katana), albeit one that has been in the Tendo family for generations.
So, in the end it appears to simply be that Kisara is an unnaturally talented martial artist and that the Tendo Sword style is much more dangerous (and fantastic) than any style that can exists in reality. So it is her martial arts abilities that allow her to cut through the gun (and the floor!) and not any special property of Yukikage.