There are many supporting characters in The Night Circus, all of whom play various roles in the narrative, so I’ll pick two: Bailey and Poppet.
Bailey is introduced as a ten year old, who enters the circus during the daytime on a dare to steal something. He meets a red-haired girl then, who helps him find a way out of the circus and gives him her glove as proof that he’s completed his dare. When he is fourteen, the circus returns to his town, and he reconnects with the girl, whose name is Poppet. The circus draws him in, even more so in contrast to the life his father expects of him: to become a farmer and take over the land. Independent and stubborn, he balks at the notion of running away to join the circus at first, but ultimately agrees due to his love for both the circus and Poppet. He later agrees to take over the circus and become bound to it, in lieu of Celia and Marco.
Poppet is a member of the circus. She and her twin brother were born on the night the circus opened, which had unforeseen effects: Poppet can see the future in the stars, while Widget, her brother, can see the past on people. She forecasts that Tara Burgess will die and that the circus will catch fire, but occasionally has difficulty with her gift—she doesn’t often get specific or clear details about what is to happen. She and her brother run a show with cats, and show Bailey around the circus.