Mercedes is a supporting antagonist during Chapter 5. She is a spoiled and selfish woman, sister of Hal and wife of Charles. She is the only female who appears in the book, not counting Alice and Molly Miller (only slightly mentioned in the beginning).
Mercedes's inexperience and apparent uselessness are shown early on in the chapter. She takes no part in packing, and consistently orders Hal and Charles to unpack and repack the sleds, changing her mind about the positioning of things or forgetting to pack things. One of the older mushers starts to say that it's better to get rid of their tent, because it is spring, and the load should be smaller, but Mercedes doesn't want to get rid of the tent. When Buck and other dogs are ordered to mush, they can't pull the sled and so Hal starts to beat them senseless, and all the while Mercedes is begging him to stop. Hal doesn't listen to her, and says that the dogs are lazy. One of the experienced mushers starts to say that the dogs need rest, but Mercedes defends her brother and agrees with him. However, the sled continues not to budge, and Mercedes hugs Buck and pleads him to pull. One of the other mushers, now extremely annoyed, say that the runners are frozen in the snow. The runners are broken out, the dogs start to run, and as they round a turn, the whole pile tips over, and belongings fall off left and right. Their dogs are stopped by other people, who say to the trio that they need to get rid of some things (their tent, dishes, canned food, Mercedes' fancy clothing). When Hal and Charles start to get rid of fall the useless items, Mercedes starts to cry and beg, generally being useless. Later, she, Hal, and Charles buy 7 new dogs.
When the trip starts, Mercedes starts to steal food and give it to the dogs, thinking that they should get more food. This decision turned out to be unwise because they had only traveled 1/4 of the way and the food had already run out. Hal or Charles start to get irritated and often fight about petty things. Mercedes often adds fuel to the fire by joining in, siding (and consequently goading) with both men. When dogs started to die of starvation, Mercedes gets tired and insists on riding the sled. Hal and Charles don't want any more weight on the sled, so they leave her behind. After traveling 3 miles, it becomes clear she has no intention of following them, so Hal and Charles returned to camp and put her on the sled. Mercedes' added weight sapped the remaining dogs' energy even more.
Mercedes, with her husband and brother, arrive at John Thornton's camp. He warns them not to go on the ice, because the surface ice was melting from the sun and rapids below. Hal doesn't listen to Thornton, beating the half-dead dogs to get them to move. Mercedes watches how Hal starts beat Buck, but she says nothing, as she is too worried about herself at this point. John Thornton hits Hal, yelling at him for abusing Buck, and she starts to scream and cry hysterically. Thornton takes Buck under his care, and Mercedes, with her brother and husband, continue on their journey across the ice. It breaks, as they were warned, and Mercedes with the others drown in the lake, John Thornton and Buck watching them from afar.