Bell, under Leon's Heavenly Eye Technique, is a Pit Fiend, but that does not prevent him from being a Great Devil.
Great Devils include all current and former Nine Prisons Lords, as well as those who form their cabinet, acting as advisors and constantly vying for the lord's position — the Dukes. And each Great Devil bears a unique appearance reflecting their evil essence... in this regard, Bell seems slightly unbefitting the title of Great Devil.
Anyway, Bell's journey has been quite legendary; he started with no noble status, merely as a twitching, mindless Lesser Demon, and rose to become a mighty Pit Fiend. Through the perfect blend of cunning and strength, he led massive devil armies, constantly waging battle against demons to gain bloody glory, and was ultimately elevated by Asmodis to the position of First Lord.
