Karoku, is a character from Karneval. He is the one Nai looking for.
Warning: Spoilers ahead.
There are two Karokus, the one with the Kafka and the one with the Circus.
The Karoku with the Kafka was revealed to be the elder brother of Karoku which is the one with the circus. Well, both of them are fond of Nai.
But both of them have difference in their personalities, the one with the Kafka has a sadistic side than the other one. Karoku was communicating with Nai in his dreams trying to manipulate Nai.
The younger brother, the Karoku with the Circus is revealed to be the original Karoku.
Though he doesn’t have much scenes in the anime nor in the manga, I’m still intrigued on how he created Nai. XD