Lilias Craven is Archibald's wife and Colin's mother. She first appears in the story when Mrs. Medlock describes her to Mary as, "a sweet, pretty thing and [Archibald'd] have walked the world over to get her a blade o' grass she wanted" (2.48). Her loveliness doesn't save her from a nasty accident, though: She and Archibald built the Secret Garden together, and it housed a tree that she liked to climb to read. When she was pregnant with Colin, the branch broke and she died of her injuries, even though Colin survived.
Now, you might think that since Lilias Craven dies ten years before The Secret Garden even starts, we wouldn't get to meet her over the course of the novel. Not so, though. She speaks to Archibald in a dream, telling him that he should get his butt back to the garden in Misselthwaite Manor.
Well, no, what Lilias actually says is, "In the garden!" (27.20) when Archibald asks her where she is. But those few words tell us all we need to know about Lilias Craven. Her voice comes with the scent of "late roses" (27.16) and with a sound "like a golden flute" (27.20). In other words, she's strongly associated with gardens, sweetness, and beauty. And her voice is also the only one (along with the real-world nudge of Mrs. Sowerby's letter) that can get Archibald to break free of his morbid, brooding ways and actually go home to find his son.
While Lilias dies long before the novel begins, we still know that she must have been quite a lady from the effects she has had on The Secret Garden's other characters. Archibald loved her truly, sure, but grumpy, people-hating Ben Weatherstaff also has a soft spot for Lilias. She's the one who teaches him to take care of the Secret Garden's roses, and when Ben sees Colin for the first time, he recognizes Colin's "mother's eyes starin' at [him] out o' [Colin's] face" (21.73). Lilias must have been something special to have made such an impression on so many people.