Ms. Sleepsack

Here is Lucas's sleepsack:
Lucas slept in his sleepsack until he was almost two, when he just couldn't fit inside it anymore. Then it became his Linus blanket; he takes it to bed with him, every night, along with puppy.

This morning when he woke up he toddled into our bedroom carrying his sleepsack and puppy. He crawled up on our bed, and at one point he dropped his sleepsack on the floor. "Get her!" he commanded us.

So sleepsack is clearly not just a blanket. It -- she -- is a fully-developed character in Luke's emerging world of imaginary buddies.