Bertha Marilla Blythe, nicknamed Rilla, is the youngest of Gilbert and Anne Blythe's (nee Shirley) six children. She is named for Anne's two mother figures; Bertha Shirley (nee Willis), her birth mother, and Marilla Cuthbert, Anne's adoptive "mother". Rilla has five elder siblings - Jem, Walter, Nan, Diana (or Di) and Shirley Blythe.

Rilla is described as a "roly-poly plump" baby in the first two books she appears in (Anne of Ingleside and Rainbow Valley), with fiery red hair that is darker than the "carrots" colour but not quite auburn, which she inherits from her mother, Anne Blythe. Rilla has the hazel eyes of her father Gilbert.

At the beginning of the book, Rilla of Ingleside, in which she is featured as the main character, she is fourteen, very vain and quite spoiled. Her brother Jem and old playmate Jerry Meredith leave to fight soon after it is announced that England has declared war on Germany, and her whole world is turned upside down. And when her beloved brother Walter leaves, she is determined to help the best she can with winning the war at home. She tried her best to hide all her feelings after Walter left.

"You would never suppose," said Irene Howard to Olive Kirk afterwards, "that Walter had left for the front only this morning. But some people really have no depth of feeling. I often wish I could take things as lightly as Rilla Blythe."

And when Walter was killed in 1916,

The autumn of 1916 was a bitter season for Ingleside. Mrs. Blythe's return to health was slow, and sorrow and loneliness were in all hearts. Every one tried to hide it from the others and "carry on" cheerfully. Rilla laughed a good deal. Nobody at Ingleside was deceived by her laughter; it came from her lips only, never from her heart. But outsiders said some people got over trouble very easily, and Irene Howard remarked that she was surprised to find how shallow Rilla Blythe really was. "Why, after all her pose of being so devoted to Walter, she doesn't seem to mind his death at all. Nobody has ever seen her shed a tear or heard her mention his name. She has evidently quite forgotten him. Poor fellow—you'd really think his family would feel it more. I spoke of him to Rilla at the last Junior Red meeting—of how fine and brave and splendid he was—and I said life could never be just the same to me again, now that Walter had gone—we were such friends, you know—why I was the very first person he told about having enlisted—and Rilla answered, as coolly and indifferently as if she were speaking of an entire stranger, 'He was just one of many fine and splendid boys who have given everything for their country.' Well, I wish I could take things as calmly—but I'm not made like that. I'm so sensitive—things hurt me terribly—I really never get over them. I asked Rilla right out why she didn't put on mourning for Walter. She said her mother didn't wish it. But every one is talking about it."

In later years, she married her old friend Captain Kenneth Ford and had two children, Gilbert and Rilla, whose mane might have been Marilla.