Euphemia (we don't know her last name) is one of the maids at Chrestomanci Castle when Cat Chant comes to live there. She is a nice girl, and nice-looking, but she speaks her mind to Gwendolen, which earns her Gwendolen's hatred.
When Gwendolen performs a spell to shift her to another world and all her analog selves along one step in the series, she also leaves a spell behind that changes Euphemia into a frog. Because her eyes, though pretty, were in fact a big goggly, the frog is instantly recognizable as Euphemia, and Cat is left in big trouble.
Chrestomanci cancels the spell and puts Euphemia back to her normal self, but Cat has a lot of trouble handling the problems that result.