Betsy Sobak is an 8-year-old girl who Claudia described as a "friendly looking girl with brown hair." Her mother is Cookie Sobak, a member of the Stoneybrook Women's Club. Her father works at Tile Corp. She also has a 23 year old sister named Pat and has an infant niece or nephew. She also has classmates at Stoneybrook Elementary School named Cici, Hillary, Justin, and Joey.
Betsy loves to play practical jokes on people, even though not everyone finds them funny. Because of this, she goes through several baby-sitters prior to Claudia working with her in #19 Claudia and the Bad Joke. Betsy put a plastic fly in a dribble glass of apple juice that Claudia drank.
Claudia broke her leg as a result of one of Betsy's jokes where Betsy intentionally had Claudia swing on a broken swing (although Betsy had not anticipated the broken leg and was horrified). This accident caused Claudia to reassess whether or not baby-sitting was for her.
Betsy orders several of her jokes through McBuzz's Mail Order Catalogue and Squirmy's House of Tricks 'n' Jokes. Other members of the BSC also baby-sit for Betsy, and they get playful revenge for Claudia's leg by playing jokes on Betsy.