In the short story The Case of the Rich Woman, Dr Antrobus is a doctor employed by Parker Pyne to execute a scheme which he had devised to make Mrs Amelia Rymer happy. For this purpose, Antobus took on the persona of one "Dr Cladius Constantine" who (according to a newspaper report fabricated by Parker Pyne) had performed experiments where he transferred souls between two hypnotized bodies.
It is not clear if Antrobus was a real medical professional. Parker Pyne only described him as a friend of his with a talent for acting.