The twin Lords of Phlegethos, Belial and Fierna are a father and daughter pair who run the Fourth layer jointly. After a failed coup that Belial was a part of, he offered to share power with his daughter Fierna to appease Asmodeus. He accepted and so Fierna became the co-ruler of Phlegethos. Both of them look almost completely human, aside from their horns and clawed fingers. Both are rather attractive with sadistic personalities: they revel in killing and bloodshed. But they are no fools and will retreat when in trouble. Belial is very much a traditional devil: he's cautious, scheming and making plans to overthrow his fellow devils. He personally trains his daughter in how to be a good devil, including personally taking a hand in teaching her about "love and pain", which is exactly what it sounds like. Bleh.
Fierna is more of a free spirit, enjoying the life of a spoiled daughter-turned-figurehead leader of Phlegethos. But things began to change when Glasya came to the scene. This thwarted Belial's plot to invade Malbolge because attacking the freshly-established Lord of the Sixth is a Very Bad Idea when you're still going to be working for her dad after you're done. His plots were further compromised when Fierna and Glasya really hit it off at a hell party. The two quickly became best friends, bonding over the rigors of having an Infernal Duke for a parent, much to Belial's chagrin. He sees Glasya as a bad influence who puts all sorts of naughty ideas into his daughter's head. This would be kind of sweet, if said ideas did not include the gruesome murder of many of his more-loyal subordinates. And for the fact that Glasya murdered Naome, Belial's consort and Fierna's mother in her plot to start the Reckoning. Fierna is rapidly becoming the biggest threat to her father's reign, and it seems only a matter of time before the one-time hellish party girl will attempt a coup of her own. Of course, given infernal ethics, Belial will probably be rather proud when she finally kills him... probably.
She's also started dating a pit fiend, Gazra the commander of the Hamatulas, who doesn't like her dad much either. Levistus is making a bid for his loyalty, but the smart money's on him turning the old fart down.