Plaintiff is a noun meaning “the person who accuses someone else of a crime in court” (usually, the victim of the crime); this word is used only in legal contexts.
Plaintive is an adjective meaning piteous—“expressing pain, sorrow, or troubles”:
Kate adopted the cat who made plaintive cries for food outside her door.
Jerry’s sympathy was aroused by the plaintive letters from the prisoners.
In connection with plaintive, you might think of the word complain; however, plaintive describes only very serious complaints—if someone complains that dinner is late, you would not call that plaintive, unless you want to mock them.