Personal defenses are the bookend of universal defenses. Where universal defenses are harder to prove, personal defenses are easier to prove, though still not too easy, and where universal defenses offer great protection, even against holders in due course, personal defenses offer protection only against ordinary holders with absolutely no protection afforded against holders in due course.
Personal defenses are generally based on instances of wrongdoing, but of a more nebulous or less damaging fashion than the instances defended against with universal defenses. For example, while negotiable instruments signed by a court-ruled mentally incompetent individual can be negated with a universal defense, if the individual has not been ruled as mentally incompetent at the time of the issuance of the negotiable instrument, then no universal defense can be mounted. Instead, the only option would be a personal defense based on the same general principle.
Similarly, some of the situations that fall through the cracks of universal defenses because they are too ambiguous, or because the defending party assumes some of the blame by having made some mistakes, will be covered as personal defenses. For more information about personal defenses and the ways in which a defending party can use them, click the link.