Symbolism, Imagery, Allegory
Most of us don't think of eating ketchup as anything to write home about—we have it on burgers and fries without even thinking—but for Kiriel, ketchup is a big deal. When he first tries it, he's so excited that he even eats it plain on a bun. Ew. Then he explains, "I decided I liked the ketchup best, and ate a bun with just that on it. Was this Gluttony? I hoped so. I certainly was enjoying it" (4.27). That's right: He wants to take his ketchup consumption to sinful levels, that's how much he loves the red stuff.
To Kiriel, though, ketchup isn't just a magical sauce that makes everything more delicious. It's part of his time as a human pushing the boundaries of what he's supposed to do. He knows he's not supposed to even be on earth eating ketchup, but the danger of it makes it that much more exciting and delicious to him. Ketchup also represents his time in a body—before borrowing Shaun's shell, Kiriel has never tasted anything before. And his appreciation of ketchup reminds us of just how novel this embodied experience is for our main man… er, fall angel.