^^I totally agree with this. Some characters changes so much from one season to an other that it lacks logic and makes it a lot less realistic and frankly, it does not make any sense most of the time.
Just look at Tina from example: When I watched Season 1 this week, I could not recognized her from Season 5. Helena Peabody is also a fine example. From Season 2 to 3, she morphed into a totally different person. I hate it when they do that.
As for Jenny, well, I guess she can be excused as she seems to be suffereing from the multiple personnalities syndrome. Yet, again, they pushed it a bit too far for my tastes.