He is a Not-Thing that copied the form, memories and thoughts of the Fourteenth Doctor.
He is time-lord presenting, brown haired with big puppy-like brown eyes and looks like he's on his 50s. Like the Doctor, the Not-Doctor is a skinny man in an oddly patterned, tight-fitting suit, whose hair stuck up at odd angles
Contrary to the Doctor, the Not-Doctor is calmer and colder, he rarely smiles but when he does he usually does to Donna and the Not-Donna in a almost fond way.