I had an imaginary younger sister called Susan. I dearly loved her. I was not an only child, but my only sibling - a brother - was 12 years older than me, so it was the same. Also, when my parents returned to London in 1945, the house in which they managed to rent rooms had a large garden and my father got permission from the Landlady for me tobe able to play out there. BUT....Only ME...no friends alowed.
amy Mum actually asked my primary school teacher about this obsession I had with Susan, and was told it was pretty normal for lonely children with good imaginations, and nothing wrong about it. Susan stayed with me for many years, my Mother was pretty good about her, getting a little upset when she picked me up at school in a downpour and found me with my coat on back to front as I was having to hold it out over Susan who did not have a coat with her!! My Dad, however, found it all very difficult and could not bring himself to kiss goodnight to her,etc. Interesting my first daughter was named Susan!!!
As I got a little older I also had a wonderful dog called Trixie -who stayed with me well into my teens. Indeed, would accompany on many a walk when I was feeling down at all right until I was about 17 years of age.
Nothing wrong with imaginary friends, think they serve an excellent purpose.