The last character in the Gang of Five in James Howe's Misfits is... unknown. The four actual gang members, Joe, Bobby, Addie and Skeezie call themselves the Gang of Five partly to confuse the enemy (which means primarily the bullies who torment them but also, by extension, most of the outside world), to keep them on their toes, as Bobby puts it, and make them wonder who the fifth member is, but primarily because they know that there must be plenty of contenders for the fifth place in their gang, lonely children who are bullied and excluded—other misfits.

The events of the book might be seen as a search for the many potential fifth members of the Gang, as Howe describes the inward-looking and intimidated group gaining confidence through their involvement in student politics, looking outwards and attempting to be recognized by their classmates as something more than the "misfits" of the title. Perhaps also the reader who empathizes with their plight may adopt for him or herself the role of fifth gang member.

James Howe's The Misfits tells the story of the Gang of Five, a group of kids who are bullied and ban together to walk the halls of Paintbrush Falls Middle School as a team. Joe, Bobby, Addie, and Skeezie are the only members of the Gang of Five—so far. They give themselves this name mostly as a joke, although they do think this leaves room for at least one other member. They know they're not the only kids being tormented at their school.

Each of the members of the Gang of Five are different from their peers, and from one another. Joe is teased for being gay, Bobby for his weight, Skeezie for his fashion sense, and Addie because she's tall and smart. Together, they represent a vast spectrum of reasons that kids get teased, especially in middle school.

There are technically only four main characters in this novel. They are Bobby Goodspeed, Skeezie Tookis, Joe Bunch, and Addie Carle. Bobby is bullied by classmates for being overweight, Skeezie is bullied for the way he dresses, Joe is bullied for his sexuality, and Addie is bullied for her height. All four of these characters are called numerous hurtful names throughout the beginning and middle of the story.

These four individuals within the Gang of Five represent common stereotypes of individuals who are often bullied in school. Together they approach this issue by running in the class election. Although they do not win, they send out an important message to their fellow classmates and their bullies. Everyone is unique, and these four characters make it their mission to end the hurtful name-calling.

You have named all four of the main characters.  They call themselves the Gang of Five even though there are only four of them.  They figure that is a good name since there is likely another social outcast like them that will need to join the group someday.  

There are other characters in the book that make frequent appearances, but I would not call them main characters.  The first would be Mr. Kiley.  He is the school principal.  Another is Mr. Kellerman.  He is Bobby's boss at the tie store.  Ms. Wyman is Addie's homeroom teacher, and it is in that class that Addie refuses to say the pledge of allegiance.  Kevin Hennessey doubles as a popular type kid who also is a bully to the Gang of Five. Dushawn Carter also functions in that capacity.  

They call themselves the Gang of Five, but in reality there are only 4 of them.  You have named them all.