Doctor Who is a somewhat unique show when it comes to the cast, because actors tend to fall into one of two categories. They are either in every episode (as the Doctor and current companion are), or they only appear in their own story. While some characters may have two or three appearances, they still seldom appear in more than one season.
Most series have a fairly robust recurring cast, featuring the friends and families of the main characters, the prominent town members in a localized show, or former series regulars that return when their skills are needed. And to be fair, Doctor Who does often allow the companions' families to appear on screen from time to time, but they are still limited to being part of the companions' lives and timeline, rather than appearing during different companions' runs.
Because of this, it's worth exploring the characters that manage to break out of the mold. They're not really companions, but they are fixtures in the Doctor's life who appear throughout the show when needed. Here, we explore the nine most prominent recurring characters, looking at who they are, how often they show up, and what they bring to the show.