The Lord of the First in 5e (in prior editions this role was held by Tiamat), Bel is the Hells' premier success story, though in truth he's effectively gotten himself promoted into a corner (no, not a corner office). He frequently uses treachery, double-crossing and ambushes to further his agenda. Bel started out as a powerful Pit Fiend general in service to the Dark Eight, the devil generals in charge of the Blood War. His campaigns against the demons resulted in success after success until he seemingly changed sides and joined the Tanar'ri. When he gained their trust he led them into a battle against the devils, only to betray his demon allies and have them all killed by his forces. He returned to Baator as a hero and became the right-hand devil of Zariel, a fallen angel and then-archduke of Avernus. He served her many centuries and eventually became her lover. This became her undoing as Bel turned her court against the fallen angel and overthrew her. He somehow managed to capture her and slowly strip her of her powers while he declared himself the new Lord of Avernus. Of course, given the fact that they are both devils and he has a long history of working as a deep-cover agent, it could just be an elaborate sex thing. Devils are big on S&M.
He prepared himself from an invasion from the other Archdukes in an attempt to overthrow him and claim Avernus for themselves, but instead he was visited by Martinet, the constable of Asmodeus. He informed Bel that Asmodeus congratulated him on his guile and skill, and was willing to accept him as the new Lord of the First, on the condition that Bel retained his position as a general to the Dark Eight. Somewhat relieved at these terms, Bel accepted. But as is common to those who make deals with devils, he discovered something a little too late: his new duties as the main overseer of the hellish "homefront" of the Blood War meant that he had less time to spend politicking with the rest of the Lords, effectively cutting himself off from further advancement in their hierarchy. The other Archdukes of Hell did not really mind this: they all saw the Pit Fiend as a double-crossing upstart and none of them were willing to accept him as an ally. The only one who sees Bel as more than a nuisance is Asmodeus himself, but even then the Overlord of Hell doesn't really think very highly of him. Funny how for all his previous scheming and undeniable martial prowess (tested daily by the claws, fangs, blades, and spells wielded by the endless hordes of Demonic invaders), Bel was left with the unenviable position of being the Nine Hells' biggest guard dog--in other words, he is in charge of the necessary, even vital, task of keeping the invading Demons out of the lower layers of the Nine Hells, but by earning this "reward" he has exhausted any intraplanar avenues for advancement in Hellish politics. If you're confused about all this, Bel's situation is similar to how a real life guard dog is useful for guarding a company's building, but guard dogs obviously can't become CEO of any organization they're guarding.
As a realist who is somewhat resigned to his lot and who knows he needs all the help he can get just to keep the demons at bay, day after day (what with the Archdukes of the other layers and even the Lawful Evil deities residing on his layer, such as Tiamat and Kurtulmak, being less-than-enthusiastic about aiding him) he is the sole Archduke of Hell who is the most willing to give anyone who can offer credible assistance to his cause a fair hearing. If adventurers approach him with offers regarding taking the fight to the Tanar'ri he is more than willing to listen. Mortals who treat him with respect and without duplicity will find a surprisingly reliable ally, unmatched in the Nine Hells. But if they try to deceive him or fail in their promises, he will display all the rage and cruelty one would expect from an Archduke. He may never be short of demonic punching bags to take out his devilish anger and frustration out on, but he is a devil through and through, and is still one of the nine most powerful devils in all Hell, regardless of his dead-end job.
...But in 5e he and Zariel swapped positions again due to him temporarily loosing his entire FUCKING LAYER to demons, so maybe there's some hope for him now.