My mother's imaginary friends all had names, the first I knew about was Gordon, an invisible man sitting at the bottom of her bed, then she was worrying about a social worker called Adrian who never turned up (and I kept asking her how she checked up on these people to find out whether they really were social workers etc. "oh but he sounded so nice on the phone"), anyone could have got into the house by pretending to be a social worker, nurse or carer. The two she didn't like were Shona and Podsnap, they were the nuisance people who filled my answering machine with their messages and told her it was full. My brother thought Rebecca was imaginary too, but she turned out to be a real nurse.
If only there'd been enough real people around, she wouldn't have needed imaginary ones.