Background
Halla, being a somewhat minor character, hasnt had much information released about her. However, one of two things could be inferred by her mannerisms as she entered the pit when she fought the Unexpectables. Because she was late to enter the pit, and because of her nonchalant attitude while entering, it can be inferred that she's either new to underground fighting, or that she's so experienced that she feels comfortable seeming unprofessional.
Relations
Salvador, Vel Rock-Fist, and Silence
Halla is an ally of these three, as they are all employed by the human who previously owned Hassrad's brother's knife. While they seem to fight well together, it is unknown whether or not their relationship is more than simply being employed by the same person.