Best introduction of a new major character in an already existing television series?
Or, perhaps more broadly, favorite major character who was introduced after the series had already been long set in motion.
Serial dramas with ensemble casts and lots of bloody turnover--such as Game of Thrones or The Walking Dead--might immediately come to mind, but perhaps my favorite has to be Douglas from the Britcom The IT Crowd:
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B.J. replacing Trapper John on M*A*S*H.
We couldn't think of a name, and she kind of looked like a ferret, so it stuck.
As she matured, the ferret-like quality went away, but we still couldn't think of a name. So we wrote down a bunch on slips of paper, put them in a bowl, and waited for her to fish one out. That is how she became Elsa.
Colonel Potter was a favorite of mine, especially liked the relationship he developed with Radar.
Oh, you said "best." Never mind.
David Pierce is enormously talented, he really can do just about anything and he was brilliant on that show. BTW, my mother was his Sunday School teacher... 
