Capitalize party designations (plaintiff, defendant, etc.) only when referring to the parties in the matter that is the subject of the document.

Is it the plaintiff or plaintiff?

If you're referencing the plaintiff in your case, the Blue Book says to use "Plaintiff." If you're referring to a plaintiff, or several plaintiffs, generally, or a plaintiff from a different case (such as one you are citing), the lower case "plaintiff" should be used (unless, of course, it's the first word of a ...

Is plaintiff's counsel capitalized?

In any event, never use all-caps for party designations. When you're discussing a legal precedent, use the and don't capitalize plaintiff, defendant, etc.