The king was a male humanoid being. Like the Borg Queen of the primary universe, the cranium and upper spinal column resided independently. Cables connected with the neural transponders at the back of the head. The skin was pale, his eyes black. The king controlled holographic displays with his mind.
When travelling in an exploratory diamond, the Borg King resided at the center of the ship, near the vinculum. The loss of the corporeal form was of little consequence: In case of termination, the individual consciousness was transmitted on a subspace carrier wave back to Unimatrix One to receive a backup body. Whether this body was male or female was not predetermined. Thus, the Borg Collective could also have a Borg Queen.
In 2371, the king was aboard an exploratory diamond investigating the Beta Quadrant. When the ship was infected by an Iconian pathogen, the king limited its connections to protect the Collective. Intrigued by the technology, he began planning an invasion of the region and initiated the ship's self destruct sequence. Before the transmission of his consciousness back to Unimatrix One, he mused whether his next body may be female. (TNG - Mirror Universe novel: The Worst of Both Worlds)