Asked this exact question as well a few years ago on a Reddit thread, here’s what I got (this is directly copied from the answers I got):

In the Comics, he is a Cherub who was loyal to Samael before the rebellion. He is the supposed to be the aspect of Joy. All around very, very, very weird character. There was a theory for a bit that Ella was secretly Gaudium, but that theory got killed when the Boo Normal episode came out.

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To add on to what the other user said, Gaudium was one of the writer's favorite character from the comics, and he was looking for a way to reference him. They ended up adding Gaudium in there as an Easter Egg.