What's a word to call a person who is involuntarily used?

One who saves from danger is a saviour (American savior).

savior = one that saves from danger or destruction

Merriam Webster

The question then arises: can a savior be involuntary?

Merriam Webster continues with an example “the policeman proved to be our savior, arriving on the scene just as we were about to be mugged”. The policemen may easily have arrived by chance, making him an involuntary savior.

Another example where the savior is most certainly involuntary (even inimical) is:

James Murdoch wants to blow up the BBC. He could yet turn out to be its unwitting saviour.

Financial Times

“Oops! Sorry! I nearly fell in the mud. You are my saviour.”

You are my (a):

Lifesaver

a thing that saves one from serious difficulty. Also, a person who rescues another from danger of death, especially from drowning. a person or thing that saves a person, as from a difficult situation or critical moment: That money was a lifesaver (www.dictionary.com › browse › lifesaver ).

Can also be used in a non-literal sense such as:

Guardian Angel

a spirit who is believed to protect and help a particular person (https://dictionary.cambridge.org/dictionary/english/guardian-angel)' or;

Knight in shining armour

https://dictionary.cambridge.org/dictionary/english/a-knight-in-shining-armour?topic=preserving-and-saving

More tentatively: http://onlineslangdictionary.com/meaning-definition-of/save-(one%27s)-butt

If it doesn't have to be possessive, maybe "solid" would work. Literally true and metaphorically fitting for the example. In this case, the involuntary use is just "using" the body as a solid.

"You're solid, thanks."

"Dude, you're solid."

"You're my solid"

Last one just doesn't flow off the tongue as well