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
They appear to be twins; of them, Yang has short red hair and Yin has longer blue hair, parted in pigtails. The remnants of their Hollow masks act as a headband, which goes vertically down the middle of their head before reaching their eyebrows. They are seen carrying their Zanpakutō which are a flaming sword and whip, respectively. They wear the same outfit, a sleeveless white jacket, red/blue elbow-length gloves, white bracelets on each wrist, red/blue-colored sock-like stockings, black Arrancar-themed boots, and short red/blue-colored schoolgirl-themed skirts.

Plot
During the ceremony to move the Ōin, Yin and Yang appear and attack the royal carriage. They battle Tōshirō Hitsugaya shortly before he turns his attention to Kusaka. Later, they follow Hitsugaya to the real world and battle Ichigo Kurosaki, but are forced to retreat.

Once the Shinigami deem Hitsugaya a traitor, they, along with Kusaka, attack Ichigo once more, as well as Rukia Kuchiki. Once Kusaka freezes Ichigo with his own Hyōrinmaru, they begin to attack Rukia together. After Ichigo breaks free from Kusaka's ice with his Bankai, they fight both Ichigo and Rukia once more. Once Kusaka begins using the Ōin, they summon hoards of Hollows to attack them, but are all shot by Uryū Ishida's volley of arrows. Yang tries to attack 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. After noticing they have lost, Kusaka encases both of them in ice, seemingly killing them. However, this is not the case, as they are seen as giant Hollows once the Ōin seals a large area within a barrier. They attempt to stop the Shinigami present within the sealed area. Yoruichi Shihōin and Suì-Fēng decide to hold them off, and Suì-Fēng manages to defeat one of them 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 Ishida Uryū quickly eliminating them before they could fight.

Fire Manipulation: Yang possesses great skill at manipulating flames, capable of quickly forming large fireballs of great destructive power. At it's 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.

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.

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.