Was the cardinal the highest leader in the Roman Catholic Church?

No, the cardinal is not the highest leader in the Roman Catholic Church. The highest leader is the Pope, who is the Bishop of Rome and the spiritual leader of the global Catholic Church. Cardinals serve as advisers to the Pope and can be appointed to lead various congregations and offices within the Vatican, but they are not the supreme authority. The Pope selects cardinals, and they often participate in the conclave to elect a new pope when needed.