JJ is tired of change. She's missed Alex over the long summer; missed having that essential sixth profiler. But, like all in the BAU, she's gotten accustomed to new people. New faces through the years, with different skills and varying degrees of comfort within the team.
She sees Kate as a fresh face. Young. Eager. She's glad for the light Kate brings but she's not looking forward to seeing the light in Kate's eventually dull.
By the end of the case, JJ is intrigued. Kate's like no one she's met before; worked with.