Board Thread:The Panopticon/@comment-1293767-20151029072618/@comment-5918438-20160111212234

I suggest swapping "guest character" language with "linking character". Well, any term really but guest character. Because Ashildr and River are both recurring, but still "guests" in each appearance according to one definition despite appearing in 2+ other stories in the same series. Remember that River is the character we're counting for the series 5 finale (unless we keep the location clause, and allow the Stonehenge location to be the link).

Anyway, characters like Ashildr and River do not appear in at least 50% of the episodes, nor do they feature in a continuous run of at least 4 episodes, so they are not the "main characters" of the series, and can be counted as linking characters for their stories.