It is unclear if Belial is Fierna's father, son, brother, or something else, but it is clear that the two are linked. They currently rule Phlegethos together instead of Belial alone. While she can't enter the material plane due to the Second Breaking, she does sometimes travel elsewhere. The city of Abriymoch is the seat of her power and she wouldn't leave it unattended for long.
Fierna is an expert manipulator and incredibly charming. Like all devils, she wouldn't lie in a deal - but she is very good at making the carrot seem extremely tasty. She's not above using every trick in the book to get what she wants but will not accept a bad deal. Even other Archdevils rarely have as much insight into the soul trade as she does.
Virtues & Personality perks
While Fierna does indeed cause a lot of suffering, many souls still consider deals with her to be some of the least unpleasant after arriving in the Hells. They might end up just serving various devils who use them for their own pleasure - but that might be better than dying over and over in Avernus. Of course, those who don't reach their quota in Phlegethos are likely to be sold for use as cannon fodder in the Blood War.
Vices & Personality flaws
Being bound by her word, like all demons, Fierna is careful about making promises and will always seek out to get something out of everything she gives. Even for very small things, she prefers to have the upper hand. A mortal might find themselves having already served her several delights before negotiations even begin. But she can't force desires unto someone who doesn't want them to the point where she can take their soul. Someone who simply isn't interested in the pleasures she can provide is safe. Fierna knows she's desirable and dismisses anyone who doesn't seem to think so as simply being wrong. Her arrogance and vanity might seem like weaknesses, but they are actually tailored to keep other devils from attacking her. She is still vigilant against attempts to exploit them.
Contacts & Relations
Besides what the familial relationship between Fierna and Belial is, if any, they are also widely believed to be lovers. They cooperate to run Phlegethos and their talents complement each other. Thay are also always at each other's throats, trying to trap the other into something that will get them killed or demoted. But Fierna has always been able to talk her way out of any situation, and so far she's been unable to get rid of Belial.