The Secret Marauder

James Potter, Sirius Black, Remus Lupin, and Peter Pettigrew. Four boys that loved to cause mischief joined together in a group called the Marauders.
Now, there was one little thing about their group. Everyone knew of the Marauders; James the arrogant leader and captain of Quidditch, Sirius is the boy who defied the odds and got into Gryffindor, Remus is the kind boy with scars from who knows where, and Peter who... cheers them from the sidelines.
Everyone knew of the five Marauders. Problem was, there were four boys that they knew of. The fifth one was a mystery, but their nickname showed up many times.
Besides, why else would the Marauders always talk about someone called Frost?
And what if the person called Frost has a secret that could change lives?