It seems that the tragic character (the "Fredo" or the "Tessio") is this Bobby Corcoran. He is part Santino's gang until it is discovered by Don Vito who gives the Italians positions in the family, but cuts loose the Irish including Corcoran.

Bobby refuses to go along with the plot to kill Vito, however he ends up at the site of the shooting and while trying to shoot Stevie Dwyer to protect Vito he ends up hitting Vito in the shoulder.

He is unable to explain himself and is thus a marked man by the Corleone Family. In fact, Santino is to make his bones on Corcoran.

I genuinely felt sympathy for the character.