Yes, Billy the Kid was a real person. He was a notorious outlaw in the American Old West. His real name was Henry McCarty, and he was involved in various criminal activities, including cattle rustling and murder. His life and deeds have been the subject of many stories and legends.
I have no clear idea. To figure out who the kid is, we'd need more information about the true story. It could be a protagonist, a witness, or someone whose actions had an impact.
Kid Kulafu is more of a fictional creation with possible influences from real life. It's not a pure, documentary-style true story. The filmmakers might have taken creative liberties to shape the plot and characters.