My niece who has Asperger's did this for years. It's often even harder to tease out in girls what's just intense creative play with characters, and what's spectrum behavior.

How are her other social skills? Reciprocal conversation? Cooperative play? Does she just want you to be interested, or does she want you to participate? Would she freak out if you banished or moved the special interest characters?

For our DS we limit access to special interests when they become a problem.

DeeDee