I have to disagree here.The idea of a sentient who lives an entire life in 9 years is intriguing. I think the character would have worked better in a single episode or a short story arc. Having her as part of the main cast only served to expose the weaknesses in the idea.
The nine-year lifespan was not a good idea. It limited the possibilities of the character, something the writers did realize and therefore her lifespan was simply forgotten in most episodes, only used in certain episodes when they bothered to use it in the story, like in "Before And After". Otherwise, it was hardly mentioned and there were also hints that her lifespan could be prolonged.
What they should have dome was to give her a prolonged lifespan and focus more on mer mental abilities, like they were in episodes like "Time And Again", "Persistence Of Vision" and "Warlord". In such episodes, Kes was at her best.