When David first meets Rosa Dartle, he considers her a generally attractive woman but notices that she has a scar running across both her lips and down her chin. Steerforth later confesses that he is responsible for this, having thrown a hammer at Rosa once when they were children. The scar is therefore significant in terms of both Steerforth's and Rosa's characters. First and foremost, the scar is a tangible reminder of Steerforth's carelessness and the harm that it causes to those around him; Steerforth never learns to control his emotions (anger included), and Rosa, little Em'ly, and others suffer because of it. Meanwhile, where Rosa is concerned, the physical disfigurement represents the emotional disfigurement she has endured as a result of her relationship with Steerforth. Rosa is deeply in love with Steerforth, but she is also painfully aware that he has grown up too spoiled and flighty to return her feelings. The fact that Steerforth nevertheless continues to thoughtlessly flirt with Rosa simply exacerbates her emotional pain, and the cumulative result of her disappointment is that she becomes a cold, sarcastic, and often vindictive woman.
The timeline below shows where the symbol Rosa Dartle's Scar appears in David Copperfield. The colored dots and icons indicate which themes are associated with that appearance.
Chapter 20: Steerforth's Home
...says that she is clever, retorts that Rosa is habitually sharp. David then mentions Rosa's scar, and Steerforth is forced to admit that he is responsible for it—not, as David assumes,... (full context)
Chapter 56: The New Wound, and the Old
...his life. Rosa continues on over Mrs. Steerforth's moans, asking her to look at the scar Steerforth gave her. Steerforth's temper, Rosa says, was the direct result of Mrs. Steerforth's temperament,... (full context)