Eustass Kid is the captain of the Kid Pirates. The reason his bounty is 315,000,000 is because of the fact that he takes any insult as an attack on his honor, and punishes anyone taunting him with death. After the time-skip, Kid's bounty would grow to 470, 000, 000 which seems to be the result of him going straight to the New World and causing havoc. He is known to have attack two of Big Mom's ships, which could be what resulted in this bounty.
He ate a Devil Fruit that gave him magnetic powers. He can attract and repel any metallic object with his arms.