A unisex name is special because it can signify both a boy's name and a girl's name.
Of all common unisex names in the U.S., only about 0.2% begin with the letter U.... Also, only 0.2% of traditional girls' names begin with the letter U, for example Ursula, compared to 0.4% for boys' names, such as Uriel. In other words, while the initial letter U is more common in boys' names than in girls' names, it is still exceedingly rare overall.
We classify names as unisex that are not just officially suitable for both boys and girls, but which have actually been used many times in the English-speaking world for both boys and girls in recent years. Some of the unisex names may have more female or male connotations for individuals, but this is often subjective and goes back to the fact that in your own circle of acquaintances, you might have a boy or man, or a girl or a woman, who has this name.