If someone is my proxy, I am their _____?

Designator would be one way.

Attendance passes and proxy attendance passes shall be printed in different colors. The pass shall separately record the names of the union member or representative and the designator. The proxies shall be numbered in serial according to the order of their sign in. During the process of proxy sign-in when it is discovered that a labor union member or representative designates more than one proxy, all the repeated designation shall be deemed as null and void. Article 5 If a designator attends the meeting in person after the proxy has attended the meeting, the designation shall be deemed as terminated.

https://laws.mol.gov.tw/Eng/PrintFLAWDAT0202.aspx?lsid=FL059410

It's bit rare, but you would be their representee.

ODO:

representee
NOUN
1 A person who or thing which is represented. rare.

Collins:

representee
noun

3. archaic
a person who is represented

Collins English Dictionary. Copyright © HarperCollins Publishers

Wiktionary:

representee
Noun

One who is represented by another.

You are their guarantor:

one that gives a guaranty

In general, a financial guarantee is a promise to take responsibility for another company's financial obligation if that company cannot meet its obligation. The entity assuming this responsibility is the guarantor.

In the case of a proxy, the person presenting the proxy is "backed by" the authority of the people who have given their guarantee that the person is speaking on their behalf.