In "The Crucible," when John reveals his true relationship to Abigail, what does he also reveals about his character and his motivation?

Your question seems a little confusing.  I think you are
asking about him revealing the truth about his past relationship with Abigail to the
court.  When he admits to the court that he had an affair with Abigail, his reasoning is
simply to expose Abigail as a fraud.  He does this in an attempt to save his wife and
all of the innocent people who are being accused and condemned of witchcraft and to
bring truth and justice back to Salem.  This reveals that John Proctor is a good and
decent man willing to sacrifice himself by exposing his own sins and crimes in order to
save others.