Of all the villains we meet in Book 2 of The Faerie Queene (and there are a lot), Phaedria may seem to be the most innocuous. All she does is row around in a boat laughing to herself and flirting with knights she picks up, and, it's true, she seems to have very little effect on Guyon. Phaedria's appearance of harmlessness is actually a big part of what makes her threatening. While Furor was an obvious embodiment of a very negative and dangerous passion, Phaedria, whose name suggests the word "glittering," doesn't seem to be an obviously dangerous presence because we don't recognize the temptations of sensuality, over-the-top sexuality, and pleasure that she stands for as easily. But these temptations are still bad, as we can see from what happens to Cymochles.
Furthermore, even though Guyon seems impervious to her wiles and flirtations, she's still responsible for separating Guyon from the Palmer, an action that has serious consequences for Guyon down the road. It's fitting that Phaedria is the one to separate Guyon from his guide because Phaedria embodies not only pleasure, but the illusion of self-sufficiency and self-pleasure.
She rides around in her boat all day, unconcerned with where she's going, talking, laughing, and entertaining herself. This image, especially for Spenser's time in which female sexuality was often threatening, has a masturbatory element since Phaedria is, in essence, playing with herself. Masturbation was often viewed negatively as taboo, suggesting both an unhealthy obsession with sexual pleasure and a dangerous belief (in the opinion of the time) that you can get along in life without other people, and without God.