Kamurcho Undercover is the 4th substory in Yakuza 0.
At Vincent, a nihilistic man warns you to not sit in the corner of the bar and invites you to have a few drinks with him. He reveals that his name is Kasuga and that he is working as a journalist investigating the shady characters of Kamurocho. He is currently looked into a gang member who is selling guns from the bar Vincent. Kasuga tells you that he wants to continue drinking with you on other occasions and will share his stories with you.
The next time you come back to Vincent, Kasuga is sitting next to the gun dealer and signals his intention to buy a gun with the order of a "whisky and water." The gun dealer asks Kasuga if he is a reporter and he denies these claims. However, the dealer doesn't believe him until Kiryu intervenes on his behalf and challenges the dealer to a fight. After fighting off the dealer, you resume having drinks and Kasuga gives you the 9mm Automatic Pistol.