Y was only seen for a short time as a lowercase letter in ‌nZ…. From o alerting everyone, after she, a and s saw the word "frick", she quickly closed her eyes in fear, and said her name, showing she may have Kakologophobia (the fear of bad words). (And F killed X)
In Y, after she saw X's dead body, she crouched, quickly looked at the sky, and said her name. After F was defeated, she and N were puzzled after seeing that F just wanted to form "FRIENDS".