Undisclosed principal
An undisclosed principal is a principal for whom an agent acts where the agent does not reveal the principal's identity. It is not - or ought not - apply where an agent has neglected to mention the agency relationship altogether. An agent who has failed to reveal he is an agent at all (regardless for whom) is better known as a principal. Or a liar.
Where an agent strikes a bargain without revealing his principal's identity, he puts the counterparty at a disadvantage, since that person does not know - and cannot know, unless the agent tells her, against whom she should seek to enforce that bargain.
The common law's rather practical response is to make the agent de facto personally liable.
See also