Duck's attempt to quit during the Pine Guard's planning session. It's clear that, through his running from his destiny, he has truly convinced himself that he's nothing special, and the idea of going up against the Abomination legitimately scares him.
Duck: "We could sit here and make all the plans we want, and then we go out into the forest, right, and we find it, and then it sees me and I see it, and then I rear back, and what the fuck finishes that sentence?"
Arc 2: The Water
Duck's fear is made even more apparent in episode 7. During their encounter with the Water, he's overcome by fear and self-doubt enough to freeze up entirely mid-battle.
The Sylphs living in Amnesty Lodge have all been exiled for one reason or another, and episode 6 makes it clear that they will never be able to go back. Imagine being cut off from your friends and family and being banished to another planet with no hope of ever seeing them again.
Arc 3: The Calamity Tree
The majority of Episode 14 is a long deconstruction where the main character's lovable quirks and jokes nearly get them all killed.
Duck's been rather consistent in being really bad at lying and generally bad at talking to people when it's not in pursuit of official park ranger business. Here his inability to effectively convince people nearly gets himself and three others killed when he can't get them to listen to him.
The jokey jetpack that Heathcliff gave Duck finally gets used and severely injures Ned just by turning it on and comes close to outright killing him.
Aubrey's magic glitches, which has previously been played for laughs. Here it causes the general store to collapse, seemingly killing Duck and the people he was trying to save. The only thing she can do is drop to her knees and go into shock, whimpering in guilt.
Also from Episode 14 we get a double whammy of character backstory.
Apparently on the night Aubrey was going to leave home, her house was broken into and a fire was started that killed her mother, a fire she now believes she caused. What makes it worse is that this comes to the surface when she's talking to Mama in the wake of her magic nearly getting one of her friends killed, meaning she's confronting the idea that she might be to blame for her mother's death while also confronting the idea that her out-of-control magic nearly killed someone again.
The other whammy comes from Ned, who hears Aubrey's conversation with Mama and realizes he was one of the people who broke into her house on that fateful night. Knowing that he's at least partially to blame for such a traumatic event for her can't be easy on his conscience, especially given that Clint specifically talked about when designing the character that he wasn't the sort of criminal who wanted to hurt anyone.
Episode 16 results in the culmination of Aubrey's guilt regarding the fire that killed her mom and nearly killing Duck in episode 14. She breaks down and asks Janelle if she can take away her powers, afraid that she'll keep hurting people with them.
Episode 19 finally reveals Minerva's complete backstory: She was the war councilor to her home planet, Miralaviniax Orbital Body 5. When her world was connected to another planet much like Earth and Sylvain, Minerva personally made the decision for her people to go to war against the invading Insectoid Aliens. However, the other world's armies proved too strong, and Minerva's people were all but completely wiped out. As a final act of desperation, Minerva had the scientists of her world create a bioweapon capable of wiping out an entire species. She delivered the payload herself, effectively making her responsible for the death of two worlds. Finally, Minerva reveals that the meteor Duck saw in his vision is headed towards her homeworld and will destroy the device that she is using to communicate with him. She bids Duck farewell and is presumably Put on a Bus for good unless she finds another transmitter.
Episode 20 shows Duck is not taking being Brought Down to Normal well. Not only does he have various freak outs throughout the episode but when Leo tosses Beacon at him and charges at him to stir a heroic reaction, all Duck does is throw Beacon back and curl up in a fetal position.
Arc 4: the Shapeshifter
Ned and Aubrey's confrontation in Episode 27 about Ned stealing the Flamebright Pendant is absolutely heartbreaking. Worse, this ends up being the last scene they have together before Ned dies.
Episode 28. Ned tackles a feral Dani and is shot in the back by a panicking Pigeon Wilson, causing him to bleed to death under the stars, the first PC in TAZs long history to die. This is intercut by a pair of goodbye letters he leaves to Duck and Aubrey, urging them to keep living and loving, and fighting the good fight.
Griffin: "And then she shouts your name and she shouts something else but you just can't hear it anymore. You just hear the wind. And you just see the stars above you. They are beautiful...and then they're gone."
When Aubrey sees Ned's body, she tries healing him, only to realize it does nothing. She breaks down into heartbreaking denial as she tries reasoning this must be the Shapeshifter because she told Ned to leave, and she's still crying when she goes to investigate what's happening to Mount Kepler.