She is Greg's aunt who always kisses him on the lips. Greg's mom, Susan, says it's rude to wipe off the kisses Dorothy gives him. She is overweight and obese, and sometimes passes way too much gas, much to Greg's dismay.
She appears on the "Have You Ever" page in the Do-It-Yourself Book when Greg asks "Have you ever been kissed full on the lips by a relative who's older than seventy?" It means she might be older than 70 years old.