Only child
An only child was an individual with no siblings.

Pavel Chekov was an only child. However, he once believed, while under the influence of the Beta XII-A entity, that he had a brother named Piotr who was killed by the Klingons. ()

Data told his brother Lore that he made him wish he were an only child. ()

Wesley Crusher was the only child of Beverly and Jack Crusher. ()

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Family