I am level 40 and i recently heard that after defeating fin dorsal 3 times and get the badge "best chum" and equip it he is available to buy as a companion i have done it but i cant find the badge i searched multiple times in my badge list, i have fought him in the last dungeon just for fun once when i was bored, could that have affected the story line.

To get the badge "Best Chum" you must be in Valencia and have done some guy's quest. I forget his name but he's located in between the Outlaws Outfiter and Remington's Steel in Skull Island.