WordNet sense 1 (a person or institution against whom an action is brought in a court of law; the person being sued or accused):

defendant

WordNet sense 2 (not as expected):

fishy, funny, shady, suspicious

WordNet sense 3 (subject to question):

questionable

WordNet sense 4 (imagine to be the case or true or probable):

surmise

WordNet sense 5 (regard as untrustworthy; regard with suspicion; have no faith or confidence in):

distrust, mistrust