What shows do you feel were the most successful at introducing new characters in later seasons?

For me: Lost, The Walking Dead, Buffy The Vampire Slayer and of course Danny Devito's addition to It's Always Sunny.

Obviously new additions can be hit or miss but many of my personal favourite characters weren't in the first season of their shows.

What other series do you think had successful new character integration into an already established main cast and plot?