Complete the analogy: Physician : Patient :: Advocate : ?

The relation is a professional : the person who comes to him and whose cause he serves. A physician serves a patient; an advocate serves a client. 'Judge' and 'Witness' are the traps drawn from the same courtroom, but the judge stands above the advocate and the witness merely gives evidence, so neither is the person who engages him. A verdict is an outcome, not a person at all.