Ao: "I see...you remembered me this entire time..."
Made incredibly worse when you realize what normally happens to Human-Coralian hybrids (if the stillbirth of Ao's sister is anything to go by).
Chief Bruno's death, as well as his conclusion about the Iwato Island incident.
Never let children die!!
In episode 8, Ao says he wants to beat up his father when he finally meets him, which brings to mind how Renton must be affected by the separation.
Endo, a Japanese soldier, met Eureka 10 years ago, where she asked him to look out for her son, Ao. In episode 13, seeing her cry out Ao's name, he remembers his promise and sacrifices himself by distracting Truth long enough to save Ao.
Worse, they place the blame on Ao.
The unceremonious scuttling of the Gekko in episode 14 is enough to bring any fan of the original series to tears.
Nirvash releasing Eureka to hold off Truth & getting trashed.
Naru leaving Ao once again.
Episode 16 Subjects Team Goldilocks to a Ret Gone by Ao's own hands. He's the only one who remembers and it is killing him.
Christophe Blanc's sacrifice in episode 19. Incredibly badass or not, Fleur had to watch her father sacrifice himself for her.
The Posiden being destroyed after we get some time with Team Harlequin realizing their impending doom. They escape, but 200 crew and family members do not.
Episode 20 has Fleur finally breaking down after seeing her father's Video Will and being abandoned by Elena.
Episode 22 has the Bad Future, where Renton walks upon a mother and child dead and decayed, wrapped close together.
Episode 24 has Ao's sister revealed to have died at birth. And Ao displacing himself in time to the point he may no longer exist in his own time period.