Baku-Kaijin, also known as the Tapir Mutant, is a monster serving the secret organization Gorgom in the series. Its background is that of a creature created by Gorgom, deployed for a specific and critical mission related to the awakening of Shadow Moon. The creature's name and design are derived from the Japanese tapir, an animal traditionally known in folklore as a devourer of dreams.

In terms of personality, the available information does not provide distinct character traits or emotional depth, as Baku-Kaijin functions primarily as an agent carrying out the orders of the Gorgom High Priests. Its motivation is directly tied to its mission from the organization: to capture Kyoko Akizuki, the sister of Nobuhiko Akizuki, in order to extract her life force. This life force was intended to be used to complete the awakening of Shadow Moon, the rival rider to the protagonist.

The role of Baku-Kaijin in the story is a pivotal one within a single episode. It is the instrument through which Gorgom attempts to secure the final component needed for Shadow Moon’s resurrection. It achieves this by using its unique powers to steal Kyoko’s dreams, using them as a lure to capture her. Once Kyoko is taken, Baku-Kaijin extracts her spirit from her body to facilitate the energy transfer to Shadow Moon. The creature’s key relationship is with its Gorgom masters, specifically the High Priest Darom, who performs the ritual of energy transfer after Kyoko is captured. Its direct antagonist is Kotaro Minami, the protagonist Kamen Rider BLACK, who confronts the monster to rescue Kyoko.

Regarding development, Baku-Kaijin shows no narrative change or arc. Its entire function is fulfilled in its single appearance, and it is ultimately destroyed by Kamen Rider BLACK, which leads to Kyoko’s spirit returning to her body. The creature is not known to possess any significant abilities related to physical combat. Instead, its notable powers are entirely based on dream manipulation and spiritual extraction. It can teleport, shoot sparks from its mouth, and emit bubbles that absorb the dreams of any person. These bubbles can bring a person's dreams into physical reality as a dream sphere. Harming this sphere directly hurts the person whose dream was taken. Baku-Kaijin uses this ability to control a victim's dreams and lure them into a trap. Following the dream theft, it can also extract a person's spirit from their physical body.