Jessy mentions Ziggy and Riga as part of a gang of kittypets and strays who like to roam and cause trouble. She claims she doesn't know them very well, but she does know where they live. She mentions him again when she thinks they are at Victor's nest, and that they're very likely to cause trouble.
When Bramblestar turns to the fence to see Victor, Scarlet, and O'Hara crowding on the fence, Jessy murmurs that Riga and Ziggy are sure to be close by. They do indeed appear during the fight against the ThunderClan cats, following Riga.
Ziggy and Riga are named after two kittens that Vicky's sister has.