In One Piece SBS Volume 104 Oda revealed that Kid, Killer, Heat, and Wire were born in a certain island in the South Blue unaffiliated with the World Government, which was ruled by a criminal gang that acted as its royalty.
The island was subdivided into four districts, which each had its own local criminal gang, each led by the four aforementioned characters, with conflicts arising on a daily basis. In spite of that, Kid and Killer had been childhood friends since they were young.
At some point Kid and Killer became close friends with a girl called Victoria S. (Shirton) Doruyanaika, who became their first crush for the both of them. This character was previously mentioned and talked about in the Volumes 87 and 98 SBS.
However, one day Victoria was killed by the gang ruling the country. This drove Kid so mad that he united the four gangs from the four districts, and led a coup d’état alongside Killer, Heat, and Wire to take down the criminal gang from power, overthrowing the current rule. After that event, Kid told the others that he “didn’t want to live in such a confined world”, so he formed a pirate crew with the four of them and set out the sea, naming their ship the “Victoria Punk” in Victoria’s memory.