As children, most of us girls (the girly ones anyway) spent hours dressing up in Mom’s clothes and styling Barbie in prefab outfits. And then there were little ones like Cassie Williams, who made outfits for her guinea pig, Punkin. (A dropped-waist, tie-dyed number for Punkin’s appearance at the fourth-grade science fair was sheer genius.) So it should come as no surprise that Cassie’s talents led her to become a designer. Cass Guy, her line of coats, dresses, skirts, pants, and tops is the most impressive debut we’ve seen in some time. Appropriate as either day or evening attire, the clothes are standouts in cut, fabric, and detail. A yellow sundress with peach and lilac accents is just the thing for a day at the beach or sunset cocktails. Her light pink shirtdress is office perfect. And her pinstriped trench with floral trim is a sure bet for rainy days. Oh, and keep your eyes open for dropped-waist styles. They are, after all, her signature. Just ask Punkin. Available at Backspace, 508 Hayes Street, between Laguna and Octavia Streets (415-701-7112). To see styles, go to cassguy.com.