User:SOTO/Forum Archive/The Panopticon/@comment-1293767-20151029072618/@comment-27501528-20160117162349

From Tardis Wiki, the free Doctor Who reference

Ah, right. I think SOTO's point here:

SOTO wrote: 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.

Sums up the solution to the previous 'linking characters' issue. Characters who don't appear in a majority of stories/a continuous run of at least 4 episodes can't really be considered series regulars, I'd say.