Plaintively tells you how something is said or sounds: in that high, sad, pleading way. A child may ask plaintively if they can stay up; a cat may meow plaintively at a closed door; a singer may end a line plaintively. It is the adverb from plaintive, so it modifies a verb, not a person as a type. The word is a little literary. In speech, people more often say “in a sad voice” — though actually plaintively is still the neat word when you want that exact colour: not just unhappy, but thin and asking.