This is the 145th most frequent Bulgarian word.
кой
"Who"
'Кой' is used to determine or specify an unknown object among others.
Не знам кой ключ отклюва тази стая.
I don't know which key unlocks this room.
'Кой' is used as an interrogative pronoun asking to identify a person.
Кой е новият ти приятел?
Who is your new friend?
'Кой' acts as an embedded question within the sentence.