Halua is an NPC in NieR Replicant. She is Emil's sister and a side character. She only has a brief appearance during Emil's discovery of the Snow White Plans, and their exploration in the Underground Laboratory below the Emil's Manor.
She is only briefly mentioned as Emil's memory when he recalls the experiments and how his abilities came to be. Some story spoilers below.
Emil and Halua were once normal young children without powers until they were used as part of an experiment in the Snow White Plans. The experiment uses magic to turn people into weapons. He recalls the creation of the perfect weapon, "Number 6" until it was deemed a failure because they had lost control of it. Another weapon, Number 7, was created to petrify Number 6 and seal it away. Emil was Number 7, and Number 6 was his sister, Halua.
After their encounter with Halua, Emil fuses with her, which changes his physical appearance for the rest of the game. Despite his new look, he gains the ability to control his powers and reverse the petrification of people.
Halua does not appear in NieR Replicant as herself. NieR meets her as Number 6, and fights her during a boss sequence. It is only when she is defeated, that a cutscene shows her true face to Emil one last time before she fuses with Emil gives him the ability to control his powers.