Monster No. 204. A hide-and-seek monster that hides in walls to secretly devour the seeker. Modeled after the Mecha Chameleon, it looks like a citizen version of one but has antennae, and its true form features numerous mouths and eyes. Its running form when chased by Merong White and Merong Black perfectly matches the Mecha Chameleon. Seeing its arms stretch while attacking Black, it appears capable of elongating body parts. As mentioned, there is no specific data so standard bullets are sufficient, but since it pushed White and Black off simultaneously, it seems to possess high strength despite low defense. Unlike the Mecha Chameleon which hides by drawing itself, this monster has the ability to blend in by standing directly against walls like a real chameleon. According to the music video and Monster Encyclopedia, it appears in famous locations; if a human (Yaya tribe) who enters that space fails to find it, it immediately devours them and uses their shadow to multiply and hunt more humans.

Gender: Male. Race: Yaya (Cat). Occupation: Hunter. Relationship: Best friend or brother to Merong White; friend to Spooky, Violet, and Yako; close older brother figure to Sadong. One of the two main protagonists of the Monster Encyclopedia series in Red Longjohns Yako. He is tasked with recovering monsters appearing on the Yaya planet and delivering them to the lab. While partners with Merong White, they have opposite traits. Notably, Black's eyes have minus sign (-) pupils, while White's have plus sign (+) pupils. Black keeps a clean desk and often engages in negative thinking, whereas White has a messy desk and is a positive thinker. When dealing with monsters, Black fights directly with a weapon resembling a pistol, while White uses his head to find clues to resolve the situation without weapons. Also, White is usually the one in charge of administering vaccine shots to the monsters.

Gender: Male. Race: Yaya (Cat). Relationship: Best friend or brother to Merong Black; friend to Spooky, Violet, and Yako; close older brother figure to Sadong. One of the two main protagonists of the Monster Encyclopedia series in Red Longjohns Yako. He is tasked with recovering monsters appearing on the Yaya planet and delivering them to the lab. While partners with Merong Black, they have opposite traits. Notably, White's eyes have plus sign (+) pupils, while Black's have minus sign (-) pupils. Additionally, White has a messy desk and is a positive thinker, whereas Black keeps a clean desk and often engages in negative thinking. When dealing with monsters, Black fights directly with a weapon resembling a pistol, while White uses his head to find clues to resolve the situation without weapons. White is also primarily responsible for administering vaccine shots to the monsters.

Gender: Female. Race: Yaya (Cat). Occupation: Hunter. Relationship: Friend to Merong White, Merong Black, Spooky, and Violet. She has multiplication sign (×) pupils. Similar to the Black and White duo, she appears in yellow, the complementary color to Violet.

Gender: Female. Race: Yaya (Cat). Relationship: Friend to Merong Black and Merong White; close older sister figure to Sadong. Relationships: Merong Black & Merong White: Partners and friends. Sadong: Someone she finds adorable. Sadong is the reason Violet stayed at Black and White's office for several days. Spooky, Merong Pink, Merong Mint: Partners and juniors. Doctor: Boss. Merong Yellow: Presumed partners and friends. Given her division sign pupils and contrasting colors, there seems to be some hidden lore. A character from Red Longjohns Yako. She is the first female hunter character introduced. Since her debut, she has become a regular in the Monster Encyclopedia, accompanying Black and White. Violet's eyes are division signs (÷), and due to her extremely taciturn nature, she expresses herself through gestures. Black often fails to understand and asks White for the meaning. Currently, White is the only one who can interpret her. When facing monsters, she uses weapons resembling boxing gloves. Since she was shown defeating a monster with a single punch in her first appearance, she is clearly a possessor of immense strength.