3- PORTGAS D. ACE
Portgas D. Ace was the adopted older brother of Luffy and Sabo, and son of the late Pirate King, Gol D. Roger and his lover, Portgas D. Rouge.
Hunted by the World Government for his lineage, Ace was captured and sentenced to death, which resulted in the Battle of Marineford in an all out clash of powers. Ace was freed, but he sacrificed his life to protect his younger brother from Akainu.
His death ultimately proved to be the catalyst leading to Luffy to train for two years to become strong enough to protect his friends, paving the way for the second half of the series.