Zee
Aside from being a letter of the alphabet, "Z" or "Zee" was also sometimes used a nickname given to individuals whose name contained the letter "Z". 

Lily Sloane told Zefram Cochrane "Z, you've had enough," after he offered her another round of drinks on the night before their launch of the Phoenix. ()

Ezri Tigan was called "Zee" by her brother, Norvo. Her other brother, Janel, called her "little girl." ()

Reginald Barclay referred to his friend, Doctor Lewis Zimmerman as "Doctor Z". ()

Dal R'El occasionally referred to Zero as "Z". ()

it:Zee
Nicknames