Warning: Spoilers Off applies to these pages. Proceed at your own risk.
Jack and Ace's brotherhood. They are the closest of brothers and understand each other better than they understand themselves. For example, when Ace is declared mute, Jacks first thought is how to help his brother through this. His second is to wonder if any of the Guardians know Sign Language and plans how to adapt to the changes. Ace also sees when Jack is upset and trying to hide it.
Fire and Ice
Young Ace's complete and unshakable faith that Jack will never harm him because he is Roger's son.
Ace's determination to get Luffy and Sabo to believe in Jack so Jack will have more people to talk to, simply because he saw Jack was lonely and did not want him to be sad.
Embers and Frost
No matter what Pitch does, he cannot make Ace fear Jack. Their bond is too strong.
Garp puts his family over his duty as a Marine in order to find and help Ace. He vows to turn against the Marines and World Government if it turns out they were the ones keeping Ace captive for the past three years.
Sabo expects Ace to be angry at him for leaving and stammers while trying to explain himself. Ace hugs him instead, silently telling him there was nothing to forgive.
Shanks tells Ace about Roger meeting the Man in the Moon. Roger isn't the mighty and scary Pirate King in the story: he's a fair, determined, and goofy captain that Shanks deeply respected. It is the first positive and good story Ace has heard about his father.