All the European city-states, in addition to several more far-flung countries including some Moorish Caliphates, Cathay, various Native American Nations and Empires, and the remote island nation of Zilmatillia, hold Embassies in Oxford, and the Ambassadors and their diplomats are frequent visitors to court. No PC can start the game as The Ambassador from a foreign Power; but such a position might be achievable through play.
The only exception to this is if the foreign Power is small or remote enough that the entire embassy staff consists of the Ambassador, for example one of the small German states. In this case requirements for the Ambassador are the same as those of Consul.
Each Embassy and its ambassadors belongs to the organisation of its home country. If you are from another country but decide not to hold any diplomatic rank you count as a level 0 member of the Embassy/Country Organisation.
Generally ranks in the Embassies depend on the country that one is from, the following is a rough guide.
Affiliation: Affiliation to a particular country is public.
This is the same 'rank' as Agent.
Requirements: A religion appropriate to the country you represent. Diplomacy, Doctrine or Languages at rank 2.
As a diplomat your duties are to assist the Ambassador and Consuls in matters of dealing with the court of the country you are in. This can often take the form of simply interacting with the nobility and other great men and represent the country. The rest of the time is your own.
This is the same 'rank' as Diplomat.
Requirements: A religion appropriate to the country you represent. Fighting or Organisation at rank 2.
Agents are employed to ensure the smooth running of the Embassy, whether this is protection or organisation. Duties usually include attending formal occasions and assisting the Ambassador in minor issues.
Requirements: A religion appropriate to the country you represent. Diplomacy at rank 3 and Languages at rank 2.
As Emissary you have greater control over the Embassy and are tasked with more important duties by the Ambassador, requiring more participation in the life of the country you are the visitor of.
Requirements: A religion appropriate to the country you represent. Diplomacy at rank 4, Languages at rank 2 and Spy Network (England) 1.
The consuls have direct access to the Ambassador and are the direct negotiators with the majority of the country leaders bar the King. It is the duty of a Consul to uphold his/her country's honour.
As the Ambassador you are charged with representing the interests of your country to England and negotiation during difficult times. Your liege relies on you to make sure that in conflicts of interest it is your country that comes out on top.