Yin and Yang

and are two loyal Arrancar girls who served Sōjirō Kusaka during the events surrounding his unusual revival and subsequent assault on Soul Society.

Appearance
Appearing to be twins, the two of them are almost identical in appearance. Of them, however, Yang is seen with short red hair, while Yin has longer blue hair which is parted in pigtails. The remnants of their Hollow masks act as some kind of headband, of which goes vertically down the middle of their heads before reaching their eyebrows. The mask remnants also act as a sheath for their weapons, sitting behind their head horizontally.

They are usually seen carrying their Zanpakutō drawn however, the two of which take the form of a flaming sword for Yang, and an electrically charged whip for Yin. They both wear the same outfit, which is reminiscent of the white Arrancar uniform. A sleeveless white jacket, elbow-length gloves, white bracelets on each wrist, sock-like stockings, black Arrancar-themed boots, and short schoolgirl-themed skirts. Of this the skirt, stockings and gloves are coloured to represent their element (red for Yang, blue for Yin).

Plot
During the ceremony to move the Ōin, Yin and Yang are sent to attack it, appearing and causing the destruction for most of the royal procession. While Yin is drawn off into a scuffle with Rangiku Matsumoto, Yang is confronted by Tōshirō Hitsugaya shortly before running into the wreckage, leading the young captain towards Sōjirō Kusaka. Later, they follow Hitsugaya to the living world and battle Ichigo Kurosaki, inflicting a severe head injury due to their combined attacks, but are forced to retreat when the Substitute Shinigami proves to be stronger then they can handle.

After the Gotei 13's final decision to deem Hitsugaya a traitor, they along with Kusaka, attack Ichigo once more, as well as Rukia Kuchiki. After a prolonged battle, Kusaka manages to freeze Ichigo with his own Hyōrinmaru, and leaves them to finish of Rukia together. However, Ichigo breaks free from Kusaka's ice with his Bankai shortly after, and they are then forced to fight both Ichigo and Rukia once more. Once Kusaka begins using the Ōin, they summon a large horde of Hollows to attack them, but are all decimated by Uryū Ishida's volley of arrows. Yang makes an attempt at attacking them by using a massive fire blast, but Chad arrives and deflects it with his El Directo attack. Ichigo and Rukia leave for the Soul Society to find Kusaka, leaving Chad and Uryū to fight against Yin and Yang.

After Kusaka breaks the Ōin, Yin and Yang return to him on Sōkyoku Hill. Noticing that they had lost and were heavily injured, Kusaka encased both of them in ice, seemingly killing them off. However, this is later proven to not be the case, as they are seen again later as giant Hollows once the Ōin seals a large area within a barrier. They attempt to stop the Shinigami present within the sealed area, but Yoruichi Shihōin and Suì-Fēng decide to hold them off allowing the others to get through. Suì-Fēng manages to defeat Yang with her Zanpakutō's special ability, but she immediately regenerates and survives, due to the effects of the Ōin. They are both finally killed when Ichigo and Hitsugaya defeat Kusaka, which dispels the dimension created by the Ōin.

Powers & Abilities
Combination Attacks: Yin and Yang are capable of combining their different elemental attacks to create more powerful techniques, such as when they fought with Ichigo Kurosaki in the living world. The resulting combination was powerful enough to equal a Getsuga Tenshō from the Substitute Shinigami's Shikai.

Sonído: Both Yin and Yang have displayed an average level of skill when using Sonído, using their speed to overwhelm the procession guarding the Ōin, and later fight on competent terms with Rukia Kuchiki and Ichigo Kurosaki.

Summons Hollows: Through unknown means, Yin and Yang are able to summon a large amount of Hollows at once, however, the level of power of the Hollow is unknown due to Uryū Ishida quickly eliminating them before they could fight.

Lightning Manipulation: Yin has shown an extensive level of control over electricity, being able to electrify her own Zanpakutō to increase its effectiveness, and fire condensed balls of lightning. Like Yang, she can also transform into her element at will, taking the form of two electrical balls that move and spin at high speeds. Unlike Yang's fireball form, Yin's electrical balls seem to be much more solid when colliding with bodies.

Fire Manipulation: Yang possesses great skill at manipulating flames, capable of quickly forming large fireballs of great destructive power. At its peak, the power of her fireballs could equal a the strength of Yasutora Sado's El Directo. She also possesses the ability to transform into a ball of fire for quick mobility, and upon contact with it most normal souls are incinerated quickly.

Zanpakutō
Yin has a long whip that generates electricity. She is able to fire electrical projectiles and lightning bolts from her Zanpakutō, as well as wield it while transformed into her elemental form.

Yang wields a sword that is can be coated in flames. She is able to fire fireballs from her Zanpakutō, as well as wield it when transformed into her elemental form.

Hollow Forms
After Sōjirō Kusaka acquires the Ōin, he transforms Yin and Yang into Hollows. They have displayed a few abilities in these forms that they did not display in their humanoid forms.

Cero: Like many high-classed Hollows, Yin and Yang are able to fire Ceros. Unlike most Hollows, Yin's Cero is composed entirely of electricity, while Yang's is composed entirely of fire. Yang's Cero appeared powerful enough to make Yoruichi Shihōin resort to using Shunkō in order to counter it, and was able to fire it for an extended period without charging.


 * They are also capable of firing much smaller blasts of their respective elements, more rapidly with no charge. However, these are much weaker than their Ceros, as Ikkaku Madarame and Yumichika Ayasegawa were able to deflect them with their Shikai alone. In this respect these blasts are similar to Bala blasts.

High-Speed Regeneration: When Kusaka transforms them into Hollows, they appear to have gained impressive high-speed regeneration traits. As demonstrated by Yang, even when seemingly killed by Suzumebachi's special ability they are capable of recovering from the damage. Later, Mayuri Kurotsuchi explains that this is due to the effects of the Ōin, whose power makes matters such as life and death become trivial.