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: Both of them can combine their attacks such as when they fought with Ichigo Kurosaki, who blocks it with his Getsuga Tenshō.

Sonído: Both Yin and Yang have displayed that they could use Sonído, both in combat and to escape from combat.

Summons Hollows: Through unknown means, Yin and Yang are able to summon a large amount of Hollows at once.

Zanpakutō
Yin has a long whip that generates electricity. She is able to fire electric projectiles and lightning bolts from her Zanpakutō.

Yang wields a sword that is constantly covered in flames. She is able to fire fireballs from her Zanpakutō.

Transformation: Apparently, they themselves can transform into the "elements" like Yang turning into a flaming ball while Yin becoming a blue disk-like shape when they attacked during the transportation of the Ōin.

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.


 * They are also able to fire smaller blasts more rapidly with no charge, but these are weaker than their Ceros, similar to a Bala blast.

High-Speed Regeneration: When Kusaka transforms them into Hollows, they appear to have High-Speed Regeneration after one of them was defeated by Suzumebachi's special ability, but this is due to the effects of the Ōin, for Mayuri Kurotsuchi states that "life and death becomes trivial," and Suzumebachi's special ability is fatal.