Zariel

The on-again, off-again Lord of Avernus, Zariel is a fallen angel with burned wings and a piercing gaze. She had Bel as her loyal servant, general and perhaps lover too. To her folly she trusted him too much, which eventually lead him to turn her court against her and imprisoning her. Ever since she remained locked up in the dungeons of the Bronze Citadel, where she is drained of her power by Bel and has her flesh cut up and fed to the lord.

In 5e however there was a change of power: by decree of Asmodeus, Zariel was to be freed and reinstated as the Lord of the First while Bel was demoted to her adviser, because Asmodeus preferred her aggressive risk-taking approach over Bels cautious and reserved one, since Bel actually temporarily lost his entire fucking layer to demons at one point. The exact reason behind this is unknown and likely to remain unexplained until the next version of the Fiendish Codex or The Manual of the Planes is released. (Told you the whole deposing plot was a sex thing.)

Then, in the 5e book Descent Into Avernus, everything went entirely tits-up. A long adventure in Hell reveals that Zariel was the way she was because of the betrayal of a group of holy knights led by Jander Sunstar of all people, and it is theoretically possible for the party, albeit very difficult, to redeem her back to being an angel, and even throw the Blood War for the Abyss and let the demons overrun Avernus. Not the status quo upset people expected.