Ruby, a young elephant, is the protagonist and narrator of The One and Only Ruby. Ruby was born on the African savanna, and lived with her mother, her brother, her grandmother, and many other elephants in a large herd. After her mother is killed by ivory poachers, Ruby is raised in an elephant orphanage but then she is forcibly relocated again when the orphanage is burned down by other poachers, who capture Ruby and sell her to an American circus. When the circus goes bankrupt, Ruby ends up in a cage at a mall. Here, she meets Stella, a kind matriarchal elephant who loves and cares for her, as well as Uncle Ivan, a gorilla, who is also in a cage, and Uncle Bob, a stray dog who slips in and out of the cages. Ruby is relocated one final time to the Elephant Odyssey enclosure in the WildWorld Zoological Park and Sanctuary. Unfortunately, Stella dies before she too can come to live at the sanctuary, a tragedy that grieves Ruby, particularly because Stella always dreamed of being part of a herd again and Ruby promised her that she would.

Ruby is a dynamic character who learns and evolves over the course of the novel.