According to the Ars Goetia, Bael is the First Principal demonic Spirit who rules as king of the East. He has command of over 66 legions of demons, who he speaks to with a hoarse voice. His appearance varies, although the most common is a three-headed apparition, with the heads being a cat, a man and a toad. He is able to make his conjurer invisible. According to the Grand Grimoire, he is a direct subordinate of Lucifuge.
Bael is a demon lord who has taken control of Tokyo after a cataclysmic bombing of the area. Pazuzu instructs the protagonist and his friend to defeat him in order to bring peace, however a witch, who betrayed Pazuzu, reveals that he is actually manipulating the two into defeating Bael so he can take over as ruler of Tokyo. The friend refuses to believe this, however, and leaves the protagonist. The witch then joins him in his place. Later on, after going through Zaratan Island, the protagonist arrives at Bael's Castle, where the friend, now known as the Dark Hero, has infiltrated. When the protagonist reaches Bael, the Dark Hero is nearly dead after attempting to defeat him. After the protagonist defeats Bael, the Dark Hero dies. Bael attempts to escape as a frog, but the hero can either kill him or catch him. If Bael is spared, he can later merge with Beelzebub to attain his true form, Baal, who joins the protagonist's party. Capturing the frog is required for the True Ending route.
Shin Megami Tensei
"Origin: Babylonia. A king of Hell, ruling over 66 demon legions. He is one of the 72 demons of the Goetia and presides over all kinds of knowledge. It's said he is able to satisfy all sexual desires. He appears with a spider's body and the heads of a cat, human, and toad. Though there are times at which he appears as a mix of these species. He is an especially powerful king in the Goetia, and 'leads the eastern army.'"
—Shin Megami Tensei A-Mode DDS dictionary
Majin Tensei II: Spiral Nemesis
Bael is fought in the Supreme Council found in the CAD of Megalopolis.
Bael is the demon form of Junya Kaneshiro's Shadow Self, although in the game he is closer to Beelzebub. He appears to be a rotund, winged version of Shadow Kaneshiro with compound eyes, and in the second phase of his boss fight he summons a robot that looks like a piggy bank to help him fight.