Learn English – A term for someone who can keep a lot of secrets

In obituaries, the terms "survived" and "survivor" are often used. I don't know of a single word to express this, but you could say that he is "the family's only survivor" or "the family is survived by only one member."

"A woman lacking cleavage" is not the opposite of "cleavage," it's the opposite of "a woman showing cleavage," which there is not a single word for, either.

Not every word--even a word that is "a feature of something"--has a single word you can point to as its "opposite." What is the opposite of "redheaded"? It's "not redheaded." Similarly, the opposite of "showing cleavage" is "not showing cleavage."

Cleavage, as it's used in American English at least, is also not an inherent feature of a woman's body; it's an effect produced in large part by the way the woman is dressed. It doesn't matter how a person's body is shaped; if she is wearing a turtleneck sweater, she is not going to show any "cleavage."

As for cleavage shown at the bottom of the breasts, impeccable source Jezebel calls it "underboob."