What's a term for what a "surrogate" stands in for?

I'm not exactly looking for an antonym, however I am searching for a general term which refers to that which the surrogate stands in for. For clarity, I am using the term surrogate to mean to "put in another's place, substitute."

The best I can come up with is "original," as in: "the surrogate comes to occupy the position of the original." The issue with "original," "genuine," "real," and "true," is that it implies a lack of originality, reality, truth, or a genuine-nature of the surrogate.

Is there a word that works better in this instance?

Any help would be greatly appreciated.

Solution 1:

If I am acting as somebody's surrogate—or as their proxy, representative, or agent—in some matter, then they are the principal of the matter:

[Merriam-Webster]
1 : a person who has controlling authority or is in a leading position: such as
c : one who engages another to act as an agent subject to general control and instruction
specifically : the person from whom an agent's authority derives

In other words, let's say I'm a real-estate agent who's involved in the sale of a house. I would act as a surrogate on behalf of the owner of the house, executing the owner's wishes in the sale. But the owner would retain control over the nature of my actions and provide me with instructions. The owner would be the principal involved in the sale. I would represent them and report to them.