Annie is Jack’s younger sister and the story’s other main character. In Sal Murdocca’s illustrations, the seven-year-old has blue eyes and blond pigtails. Annie enjoys make-believe and is comfortable with magical explanations of the phenomena the children experience in the tree house, while Jack longs for concrete facts. The boisterous Annie often takes bold and impulsive action. Though Jack repeatedly urges her to be more careful throughout the story, she’s the first to climb into the tree house at the start of the book, and she takes a risk by making a wish on a picture she can’t clearly see near the end of the story: “I hope it wasn’t the dinosaur book!” (52). Osborne sometimes uses the contrast between Annie’s daring and her brother’s caution to provide humor, such as when Jack tells Annie that they should formulate a plan before entering the medieval castle: “‘We need to examine the situation,’ said Jack. ‘From home.’ ‘Let’s examine it here!’ said Annie” (14). Annie’s imagination, daring, and impulsivity make her eager for adventure and balance out her brother’s character traits.

As the other main character, Annie also contributes to the story’s structure and themes. Because she’s more intrepid than Jack, Osborne uses her to initiate much of the plot. Annie sets the story’s events into motion when she goes to her brother’s room before dawn and suggests, “Let’s go back to the tree house right now” (5). In addition, she decides the main setting by selecting the book about castles and making a wish on the knight’s picture. This story focuses more on Jack’s development, leaving Annie’s characterization largely static. However, there are indications that she’s beginning to exercise caution. For example, she urges Jack to hide before he puts on the helmet, in a reversal of their usual behavior patterns. In addition, her cooperation with her brother improves throughout the story, advancing the theme of the value of teamwork and problem-solving. She also develops the theme by saving Jack from crocodiles and by using her imagination to secure their escape from the dungeon. Annie also contributes to the theme of the excitement of discovery because she joins her brother on magical adventures across time and works with him to unravel the mystery of “the magic person” who created the tree house (3). Her example also encourages Jack to embrace the thrill of learning through direct experience, such as when she charges into the castle and readily accepts the knight’s help. With her characteristic enthusiasm, Annie propels the plot of the adventure story, giving the author opportunities to explore the themes of teamwork and discovery.

The knight is the supporting character who gives the story its title. Osborne portrays the knight as a striking and gallant figure, as his sudden appearance at the end of Chapter 8 illustrates: “The knight sat on the black horse. His armor shone in the moonlight. A visor hid his face, but he seemed to be staring straight at Jack and Annie” (47). As this excerpt suggests, the knight is a highly enigmatic character. The children don’t know what he looks like because of his helmet, and he has no dialogue in the story. Adding to the layers of mystery around the knight, Jack and Annie learn in later novels that this was a form assumed by the shapeshifting enchantress Morgan Le Fay. 

Part of the knight’s narrative function is to be a symbol of the romanticized picture of the Middle Ages popular in fiction. Indeed, the imaginative Annie likely has this idealized Age of Chivalry in mind, rather than the historical realities of life in the Middle Ages, when she selects the book about knights and castles and declares, “There’s my favorite” (11). The character’s actions uphold the knightly ideals of kindness and heroism when he comes to the children’s rescue and helps them find the tree house. The knight’s protectiveness toward the children aligns with Morgan’s broader role in the series as the children’s mentor.

Osborne uses the knight to support the story’s genre, structure, and themes. He contributes to the novel’s genre both as a piece of historical fiction set in the Middle Ages and as a work of fantasy because he is a classic medieval knight, and his true identity is the enchantress Morgan Le Fay, a figure from Arthurian legend. The knight facilitates the story’s happy ending by helping Jack and Annie find the tree house after they become lost in the fog and by appearing in the inspiring dream that Jack has at the end of the story: “Jack dreamed they were with the knight again. All three of them were riding the black horse through the cool, dark night” (57). The thrilling experience of riding on the knight’s steed represents a milestone for Jack’s development and the themes of the excitement of discovery and overcoming fear. Additionally, the timing of the knight’s appearance in Chapter 8 supports the theme of the value of teamwork and problem-solving. The knight comes to the children’s aid after they evade the guards and escape the castle on their own, preserving Osborne’s message and the siblings’ growth. The knight’s actions fit within Morgan Le Fay’s pattern of allowing Jack and Annie to have educational and empowering adventures with her magic tree house, only intervening when necessary so as not to stifle their opportunities for learning and growth.