Babel was the largest portion of the original King of Bel's power. Belberith attempted to summon it to become the new King of Bel, but the tower failed to react to him, only revealing its true form later to the Protagonist.
Babel appears in several forms, each with a distinctive feature depending on the storyline you face it in. In 'Kingdom of Saints' with Amane, Babel's initial face when the MC summons it during a cutscene, is that of a small child. In Gin's/Haru's Path, 'Song of Hope', it appears as an old man with a long beard. In 'King of Demons', with Naoya and Kaido, it appears as a grown woman with long hair. In 'Silent Revolution', it's appearance is that of a middle-aged man with short hair.
Regardless, its abilities are the same. If the player were to succeed in defeating Babel's first form (If Babel is close to losing, its face will begin to make small twitches in and out of battle.), it will then reveal its true form - a tower-like demon with many eyes - and split itself in half, revealing the "King's Gate".
It is advised that you constantly remain in battle with Babel instead of stalling outside of it. Babel is shown to use Calamity of Babel, a Mystic attack which causes damage to every party on stage while also inflicting Mute to them. Often when Babel casts Calamity of Babel it also shifts its weakness - first to Force, then to Physical attacks - while covering its previous one.
Babel is the final Bel in the game, and the strongest boss (barring bonus boss Lucifer) in the storyline.
In this route, after defeating Babel's Tower form, the player can immediately move to and attack Babel's second form, albeit without any demons (including Black Frost). Babel now is capable of shifting its weaknesses too, though it has its power reduced and several abilities removed to compensate for the player's lack of demons.