relative clauses - "a patient who is" or "a patient whom is"?

I am still very confused on when to use who and whom, I understand the idea these sentences are correct:

• He is the person who won the competition.


• That is the person whom I went on holiday with.

But what would be correct in the following sentence:

This image shows a patient who is... or whom is?

Would it be who because the patient is the subject, or is it?