While we don't see a lot of this character, we sure hear a lot about her in Book 2. In fact, it's hearing the horrible things she did to Mordant and Amavia (see Amavia) that makes Guyon vow to find and punish her.
Indeed, even when we do come face-to-face with her at the end of the book, she says nothing, but wields her power through the seductive temptations of her hang-out, the Bower of Bliss. Her name means both "giving in to desire" and "badly mixed," and her name is itself a kind of bad mixture of a Latin word and a Greek one.
As the major villain of Book 2, Acrasia represents the opposite of all things temperate. She represents lust, uncontrolled desire, and giving in completely to sensual pleasures. Sadly, it's no accident that the embodiment of sexual temptation is female. This is a trend that goes back to Eve herself (yes, of Adam and Eve) who notoriously gave into Satan's temptation and ate a very off-limits apple. Much like Adam in the account from Genesis, men are understood in Spenser's text to be victims of Acrasia's temptation, rather than perpetrators themselves.
In addition to this Biblical association, Acrasia is also associated with the witch Circe from Homer's Odyssey. Circe famously turned men into pigs, and Acrasia also turns men into wild beasts. Circe was also infamous for delaying Odysseus and crew at her palace while an entire year actually passed by without them knowing it. Similarly, Acrasia distracts and detains knights, like her current BF Verdant, and prevents them from doing their knight thing.