Who or what is a difficult person? It's anyone who leaves you feeling upset or let down, frustrated or angry, humiliated or confused, drained or despairing.
A difficult person can be someone who behaves in an exploitative or unethical manner; they may be someone who creates a sense of distrust because they avoid saying what they really think or feel.
A difficult person may be someone who refuses to cooperate with you. They may avoid taking responsibility and duck out of commitments.
A difficult person can also be someone who is negative and critical; they find fault easily without offering any constructive or helpful alternatives.
Whether it's a manager who keeps moving the goalposts, ...