Talk:Seddie/@comment-24774793-20150430070619/@comment-14284535-20150430225705

Her role is what I used to call "as adult as a typical companion" on Doctor Who - the young twenty-something (usually female) who was mature enough to function as an adult but is immature / inexperienced / naive enough to function as a clear subordinate as a child.

This allows for multiple plot types for the character: strictly supporting the Doctor in the main plot; the main focus of a plot whom the Doctor has to rescue from a hokey cliffhanger; a lead-type character who can carry a second strand of the plot. In this case, she is a senior character and therefore likely to be a leader, be more intellectually and emotionally mature, etc., but still young enough to make "young and dumb" mistakes.

Aside: there is actually a semi-plausible scientific reason that only people older than 21 or so would die from a virus or plague of some sort. For instance, a virus may only be able to propagate within a host because of the biological changes from the cessation of bone growth.