Most kids have imaginary friends. It is part of being a child, to explore the world and see the magic there, and to provide onesself company when it is needed. This is part of the blur between reality and fantasy that makes childhood so fascinating to adults, and so fun for kids.
You may recall that Andy has an imaginary friend, T-Rex Dinosaur. We have added to the menagerie of imaginary friends, but none is so prominent, and only one other leaves the house (that would be Piggie... long story).
You may also recall that I didn't know if Joey had an imaginary friend. I am now proud and pleased to report that we know, without a doubt. Joey has imaginary friends. Note the plural.
In fact, he has an imaginary class. They are named Aaron, Eric, and Sophia. Joey is the teacher, and these three are his students. They do math problems. They have reading assignments. When they misbehave, Joey corrects them and tells them how to behave correctly, and why what they were doing was wrong. If he's a little harsh with them, they get upset and he has to apologize and comfort them.
Lots of insights to be had into his world, I think. I'm paying close attention, never fear.