Beirut wears a long black robe. He has long black hair that flutters in breeze and a long black beard that hangs down to his chest. His eyes are very small but they look as lively and energetic as the stars.
The first time we learn something about "Beirut" is when Bebe remembers that the first words he has heard after his birth are "Beirut Clan". We learn later that it is name of his ancestor, Beirut.
Beirut is the first and one of the only two Supreme Divine Beast Godeater Rats ever known of, along with his descendant Bebe. Being the only one for millions of years, he chose to name his race "Godeater Rat".
He is known as one of the most powerful beings in the Infernal Realm, one of the four Higher Planes, aside from the Sovereigns. He became well-known because of his overwhelming strength and the slaughters he has been through.
Later, when rescueing Linley and Bebe from the Okerlund Plane after having obtained all three Overgod talismans by travelling through the Chaotic Space, it is revealed that his Yulan body was in fact that of a Lesser Wind Sovereign.
He protected the Four Divine Beast Clan from the Eight Clans, and is feared by both sides.
Since Bebe is the second Godeater Rat in existence, Beirut really dotes on him, going to great lengths to help him.