Koku was a servant of the Baba Yaga that takes the appearance of an short elderly Asian man with a mustache and tail.
During the Giurescu Affair, Koku presents Grigori Rasputin and Ilsa Haupstein with an Iron Maiden. After Ilsa is entrapped in the torture device, Koku disappears underground, just before warning Rasputin "...But you be careful out there Grigori... too many humans."
More then six years later, in "Darkness Calls," Koku returned to the normal world, this time acting as a messenger between the Baba Yaga and the newly gathered witches council. Here he conveyed Baba Yaga's request that the witches offer Hellboy to her so that she could destroy him. The witches agreed, and Hellboy was sent to the Baba Yaga's faux Russia.
Koku stood by the Baba Yaga's side as the witch used all her resources to destroy Hellboy, standing mostly silent for the majority of the time. It was only in Baba Yaga's final moment of desperation, in which she was about to free the crumb of Rasputin's soul from its acorn prison, that Koku took action and intervened, throwing the acorn into the roots of the World Tree while saying "not him." The acorn then fell into Hell.
Koku is last seen standing with concern at a depowered Baba Yaga's side.
Powers and Abilities
Koku is the Baba Yaga's chief attendant and possibly her friend. After the loss of Baba Yaga's eye that trapped her in faux Russia, Koku became the Baba Yaga's chief messenger into the normal world. In Wake the Devil Koku appeared from a hidden trap door built into an ordinary looking boulder, indicating that he either knows how to manipulate reality to escape faux Russia or knows of many secret passages that exist between that world and the normal world.
Koku is apparently one of the few beings that the Baba Yaga tolerates to speak plainly to her. Koku demonstrates this by grabbing the acorn containing Rasputin's soul from her and throwing it away, an action that the Baba Yaga, surprisingly, doesn't verbally protest. However, it is also possible she simply didn't have the strength to do so at the time.