The Babylon Guardian (called Angelus the Gatekeeper in Japan) is the final boss and only boss in Sonic Riders. Somewhat confusingly, "Babylon Guardian" is also the name of the stage in which he appears in. He is also referred to as "the guardian God of Babylon" when choosing his stage.
HistoryAfter getting the Key to the Babylon Guardian back from Sonic, the Babylon Rouges enter Babylon Garden to find the Treasure of Babylon. Just as Sonic and friends are about to leave, a loud roar emanates from the palace. Sonic and friends then decide to join up with the Rogues to see what's going on. A large creature forms and reveals itself to be the Babylon Guardian. He then attacks them because he believes them to be greedy. After Sonic and his allies defeated him, the Babylon Garden disintegrates into sparkles in anguish and relinquished the Treasure of Babylon.
Role in the Archie Comics
In the comics, he is now known as Angelus the Guardian. He's a hologram made by the Ancient Babylonians used to protect the Key underneath Gigan Mountains, but Jet the Hawk, Storm the Albatross, and Wave destroyed his projectors, causing him to vanish.