She is Beverly Marsh's mother and Alvin Marsh's wife. She eventually succumbs to her fatal cancer.
Elfrida is a woman that works long hours as a waitress at the best restaurant in Derry. She often brings home unsold leftover desserts for her husband's lunch the next day. She looks older than her years, although she apparently enjoys nightly sex with her husband.
She loves her daughter Beverly very much. As she notices how Bev's body is starting to develop, she becomes more and more worried subconsciously that her husband is touching Bev in a sexually inappropriate way. She is relieved to see Bev's confusion when she asks if her father ever touched her. She clearly knows that her husband beats Bev, but she lets it go as long as it doesn't cross over into a different kind of touching.
Elfrida had dark brown hair and pretty blue eyes.
Elfrida's greatest fear is going "on the country", which is why she works so hard without complaint and appreciates that Al works hard as well.
It: Chapter Two
In the 2019 film, during one of Beverly's flashbacks including her father Alvin Marsh, Alvin says that Elfrida died giving birth to her, implying this contributed to his abuse of her.