Cassie is a cat that first appeared in "New Friend on the Block." She is the title character and a gray cat.
She is voiced by Maia Filar. In fact, at one point, she mentions a similar quote to the catchphrase of Phoebe Terese from The Magic School Bus, another character of Filar's: "In my old neighborhood, we didn't have a beach."
Cassie appears in "Practice Makes Perfect," revealing that she attends the same school Huckle and Lowly attend and has Miss Honey for her teacher, likewise.
Then she appears in "The Sleepover" as one of Sprout's guests.
Again, she appears in "The School Dance" as one of those attending the dance, ultimately dancing with a reluctant Huckle, and in "The Winners," as one of those joining the race.