Halua, also known as No. 6, is the older twin sister of Emil.
In 2026, the National Weapons Laboratory brought the twins to to their facility when they were children. Scientists within the facility experimented on them and other children with hopes of creating the ultimate weapon against the Legion. The codename for their experiment was Project Snow White.
After excessive experimenting, the researchers selected Emil to be their ideal candidate for Number 6, a skeletal monster and the ultimate weapon armed with powerful magic. Halua wanted to protect her brother from the painful remodeling process and volunteered to go in his place. She kept her human heart after the remodeling, which unfortunately caused her to go berserk in her inherent desire to keep Emil safe. Halua could not be controlled by the researchers, which led to her bolted-and-chained containment. Against her wishes, Emil was remodeled into No. 7 to be her countermeasure.
Emil and Nier, searching for a way to cure Kainé's petrification, find No. 6 in the underground labs. They fight No. 6, and she is subdued enough for Emil to petrify her. In her final moments, her soul regains its sanity and she accepts her petrification by him. Their bodies fuse together, granting Emil all her power and also transforming him into a skeletal creature.
Players can potentially obtain her head through the NieR RepliCant × SINoALICE collaboration, which also unlocks the Boy Emil/Mage job class. The weapon story for her head is a series of National Weapons Laboratory experimental recordings that were done on the twins. Identified as "Test Subject: Elder Sister," it's recorded that Halua had tried to comfort Emil during their experiments. Before she underwent the "ultimate weapon" experiment, she had written a letter expressing her wishes for his safety. The scientists were unclear over whether she had any conscious thought after the experiment and deemed her too unstable to use. They then decided to operate on Emil.
Her name is the second answer for the collaboration's Twitter quiz.
- In the early development stages, there were plans of her being a party member instead of Emil.