No its a lead hero. Frodo is the lead hero.
yes he has something to do with Davy's woman who is Calypso(now Tia) She left Davy about 300 years before Jack was even BORN...Jack is NOT the reason why Calypso left Davy unless Jack has a time machine that can transport him back300 years or so...maybe he bumped into doc from Back to the future and asked him to borrow the Delorian 馃檪
secondly The reason Jack owes a debt to Davy Jones is in DMC itself he owes him his soul because he traded it to Davy 13 years before in order to get the Pearl and now his time is up..Davy wants his soul..seriously you must have been watching the B version of DMC to come up with that crazy ass theory