What if Cecil was a form of disembodied voice with the capabilities of creating an image to represent himself, and what caused of the image affected everyone differently? The image has the potential to become solid depending on how he chooses to bend the matter around him into an image. Thus Cecil would have no solid image—hence why he can’t really be described—but can still come in contact with other beings. Though when he’s alone, or not, just whenever he chooses, he has the ability to vanish because of his lack of a physical body.
Carlos and his team didn’t want to offend Cecil in case he was in fact absent physically(I mean, going around telling people they aren’t real physically is a little insulting after all), so they surveyed the residents of Night Vale. They all gave slightly different descriptions, some vastly different, leading them closer to the conclusion that their hypothesis was-to some degree-correct, but everyone was certain that he was just as solid as the rest of them. The only one to admit that he had thought that maybe the Voice of Night Vale was literally that-a voice-was Steve Carlsberg. The man had cautiously asked Cecil about it once, and the reaction he had received made him think that maybe he was right(and also caused him to be loathed by his fellow Night Vale citizen). Considering it was not solid data to go off of and merely conclusions, the scientists didn’t get much to go off of to bring the hypothesis to any definite conclusion. A couple times Carlos tried coming in contact with Cecil to see if he was in fact real, and came to the conclusion that he was really there—at least to some degree. What kept him from trying to advance their relationship when he began to develop feelings for the radio host was the fact that they couldn’t come to any real conclusion, and the option that Cecil could just up and vanish, or worse: stop existing on him made him hesitant. He had known death would end his own existence before—he’s not stupid after all, but nothing bad ever seemed to happen to him in Night Vale, giving him a subconscious feeling of being immune to the oddities of the town. The incident in the pin retrieval area of lane five in the Desert Flower Bowling Alley and Arcade Fun Complex made him remember that Cecil wasn’t the only person who could—in a technical sense—disappear if they were to form a relationship. Thus he went for it. And thus they are cannon. Which is wonderful. And I love them.
Kinda long weird ass head cannon, but, hey, it’s Welcome to Night Vale. Everything’s weird in this fandom.