Dorothy is the great aunt of Greg (likely great-aunt due to her age), Manny and Rodrick. She is very affectionate and always kisses Greg on the lips when greeting him, making Greg find the nearest object to wipe his lips with. Dorothy is the sister of Nana Heffley and Charlie Heffley.
↑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.