"Main" characters refers to those who are apart of the "Core 5" (More information on that below.) "Recurring" characters are those who appeared in more than one issue in a series. "Guest" characters are those who appeared in only one issue in a series.
"Core Characters" are any characters within the series, whether they started out that way or were bumped up after time, who at any time were considered "main characters." Meaning that while they weren't in every single issue, they were in most. With the exception of Series 4, each series had only five core characters. Mrs. NóName is the only main character not to be considered as a part of the "core 5," seeing as that title is only given to student characters.
"Secondary Characters" are characters within the series who were never considered to be in the core five at any point.