Chikou is Mukuro's "father".
He treated her as a slave and raped her every birthday, to the point that Mukuro covered herself in acid, in order to repulse him, making him throw her out.
Still Chikou made it so that whenever Mukuro thought of him, she would see false images of him playing with her.
Using the Pseudo-Creature plant, Hiei bonded him to the plant. The plant would constantly heal any wound dealt to him unless his neck was cut off. Hiei told Mukuro to torture him as much as possible, and kill him once she grew bored of him, and said in English "Happy Birthday."
Though he doesn't make an appearance in the anime, he can be seen in the sequel OVA Eizou Hakusho in Mukuro's room, showing his fate was the same in the anime as well.