When a complaint is filed in court, there is often an allegation that the individual (non-corporate) parties are sui juris. What does this mean?
If it is alleged that defendant John Doe is sui juris, it simply means that he is not under any legal disability and has the legal capacity to manage and be responsible for his own affairs. This is a shorthand way of saying the defendant is not a minor, is not incompetent, and is otherwise subject to the power of the court to render a binding judgment on him.
If a person is not sui juris, his or her interest in court must be represented by a guardian or other personal representative having full legal capacity.
— Kevin Palmer