You don't have a column returned from your stored procedure named "FullName" is the problem. That's why you get the error.
If your stored procedure returns FirstName and LastName you need to store those in their appropriate properties.
I would expect you to have a method that populates your class from the database... and in it something like this...
FirstName = reader.GetString(reader.GetOrdinal("FirstName")));
LastName = reader.GetString(reader.GetOrdinal("LastName")));
That would populate your FirstName and LastName in your class.... then your FullName property would work since it just concatenates FirstName and LastName.