He is the son of Gold Roger, the deceased King of the Pirated and adopted older brother of Monkey D. Luffy, the main protagonist. He has the power of fire because he ate the Flame Flame Fruit. This also gives him the nickname, Fire Fist Ace. Though not related by blood, Ace and Luffy had a strong and loving bond, though they did fight often, with Luffy admitting to his crew that Ace was much stronger than him even before Ace ate the Flame Flame Fruit. Ace also showed incredible loyalty to his captain Whitebeard and the Whitebeard crew, being the second-in-command.
Eventually, the World Government discovered Ace's heritage and is able to capture him and sent him to the prison Impel Down for execution. Luffy breaks into the prison and is able to rescue his brother, but during the battle of Marineford, Ace saw Akainu, an admiral prepare an attack meant for Luffy. Ace got between the two and took the blow, Akainu's lava punch overpowering Ace. This ultimately killed Ace, which for the first time in the series, caused Luffy's spirit to be completely broken.
It also encouraged Luffy to train for 2 years to be strong enough to protect his crew, which set up the second half of the series.