In the Nyakuza Metro DLC, there is a hidden area that can be accessed in the Green Clean Station by jumping off of a moving subway train at the last second and then wall-jumping up to the top of a large, green, three-tiered LCD display above the station ledge. At the top, a cage with an egg can be found, but just near it is a Springboard that will send Hat Kid to a hidden, burning, grated area through a manhole. Upon interacting with the manhole, a message can be found on it that reads "Kiwami means extreme.". Suddenly, an eye-patched Metro Cat named Meowjima will ambush you by jumping out of the manhole and screaming:


This initiates a secret mini-boss battle. The message on the manhole is a reference to the tagline of the game Yakuza: Kiwami, and this character is a parody of Goro Majima based on his appearance in that game as well as a reference to one of its side quests "Majima Everywhere", where he ambushes Kazuma Kiryu through disguised encounters as a form of training. There is no reward for completing this boss battle, and upon interacting with the manhole again you will be sent back to the Springboard you used to enter it.