I think that Kendra, aside from establishing that Buffy's being temporarily dead created a new Slayer, sort of illustrated what Slayers were like traditionally. Her few references to herself and her training indicate that her Watcher or the Council virtually bought her as a child from her family and raised her with the single goal in mind, that she would one day become the Slayer, and she was, literally, a "by the book" Slayer, as opposite Buffy's character in her own way as Faith ever would be. Sadly, we got very little in the way of insight beyond those few facts, before she was killed. Kendra COULD have been a quite interesting character, and it's a shame that she had to be basically nothing much more than cannon fodder in the war between Mankind and the demon world/