adversary
person who is against another; opponent; enemy
ally
person (or government) that joins with another for a special purpose;
helper; supporter; associate
creditor
person who is owed money
debtor
person who owes something to another
defendant
person who is being sued or accused in a court of law
plaintiff
person who brings a lawsuit against another
optimist
one who is cheerful and expects everything to turn out all right
pessimist
one who expects things to turn out badly
predecessor
person who preceded another in a position or office
successor
person who succeeds (follows) another in a position
or office