Because the main characters are A and Rhiannon, the minor characters are everyone else. Although there could be a debate whether Justin is a main character (in that Justin is the body A inhabits when he falls in love with Rhiannon) or whether Alexander is a main character (in that A realizes that Alexander Lin is really the kind of person Rhiannon could be happy with), A is really the important character during those two 24-hour periods. Justin, of course, is a teenage boy who is dating Rhiannon, but they are having a rough time with their relationship.  

Another minor character, Nathan Daldry, is the person A inhabits immediately after Justin. Due to the fact that A leaves Nathan's body in a car at a party, Nathan realizes there is something fishy going on and tries to figure it out. Another minor character is the character of Ainsley, a teenage girl who has many allergies and a love of dogs. It is in Ainsley that A meets yet another minor character: Poole. Poole is the minor character who claims to be priest, but is really another spirit-being like A. The only difference is the spirit in Poole seems to be evil, suggesting that A should learn to stay in a body for more than one day.  

Other bodies that A inhabits can be considered minor characters as well (even though they don't necessarily further the plot): Leslie Wong (her brother Owen and her friend Carrie), Skylar Smith, Amy Tran, Roger Wilson, Margaret Weiss, Megan Powell, James the football player, Kelsea Cook, Hugo, Ashley Aston, AJ, Adam Cassidy, Chaville, Valeria, Sally Swain, Daniel Stevens, Orlando, Dylan Cooper, Vanessa Martinez, George, Surita, Xavier Adams, Dana, Michael, Vic, Marc, Vin Taylor, Lisa Marshall, Kasey, Darryl Drake, Zara, Howie Middleton, and a nameless drug addict are all examples.