Girl in yellow with blonde hair is possibly one of the sexy witches.
Sexy Witches are one of the monsters kept in The Facility. They are seen on the whiteboard with the Archives Department betting on their release.
Physical Description & Attributes
The Sexy Witches are similar to the Witches, but more conventionally attractive as opposed to the ugly, hag-like Witches seen during the Purge. The only one shown can be seen in one of the cubes, appearing as a normal-looking Caucasian woman with blonde hair, a yellow shirt, and black pants.
Their powers are unseen during the film, though presumably they are similar to that of the Witches, possibly using her feminine charms to trick or seduce others in addition to wielding black magic.
Inspiration
Sexy Witches could be considered a standalone horror trope with such notable appearances as the four girls from The Craft, Angelique Bouchard from Dark Shadows, The Witches of Eastwick (which also featured actor Richard Jenkins, who played Sitterson), Sarah Sanderson from Hocus Pocus and Morgan le Fay from the Arthurian Legends. Notable non-horror examples would be the Halliwell Sisters from Charmed, Sabrina and her aunts in Sabrina, the Teenage Witch, Samantha Stephens from Bewitched , Willow Rosenberg, Tara Maclay and Amy Madison from Buffy the Vampire Slayer, or Hermione Granger from the Harry Potter film series.