Celia works in Hahp's dad's household as a cook and servant. We know from Hahp that "She was always up early. She sang softly, almost constantly, as she fed fires, kneaded dough, ground herbs, made pies" (2.6). She seems to be one of the few good people from Hahp's home life.
The fact that Hahp's dad is such a jerk seems to impact her, too. Hahp remembers that Celia was strangely absent from the kitchen one day: "She didn't come into the kitchen until much later, her hair undone and her cheeks flushed" (26.10). The fact that Hahp's mother was also crying around this time probably means that Hahp's father was taking advantage of Celia whether she wanted to or not.