Yuzu's reaction to everything seems to be to smack it with something...usually a paper fan/harisen. Including her own father.
The facial expressions, just, the facial expressions.
When Yuya is defeated by Yuzu, he gets thrown into the air, slams straight into the sky, and lands upside down in a bush, with only his legs visible. Then the bush fades away, but he's still practically faceplanting in the same position.
When Yuya's sleeping in class muttering the words "Pendulum Summon," you know how this is going to end.
Poor Entermate Whip Viper gets cartoon-style flattened twice in the same turn, and the second time, it's clearly going Oh Crap! Yuya himself ends up faceplanting into a wall during one of these incidents.
Shingo attempting to outrun the explosions of his own monsters' destruction, only to be tossed around like a rag doll.
The montage of Sora trying to talk with Yuya was hilarious. Highlights include Sora appearing under Yuya's desk during class. One had Sora finding Yuya on the toilet. Yuya wasn't very pleased.
Yuzu pretty much derailing all of Sawatari's dramatic speeches in the guy's second appearance.
And on that note, all of his dramatic speeches.
Here's a few from episode 8:
No one bothers to pay attention to Shouzou's summoning lessons, the highlight being Sora interrupting it by eating two donuts, comparing it to Fusion Summoning. Shouzou is not pleased.
Shingo's lackeys claim that thanks to "Yuya" ( actually the Dark Duelist), Shingo has suffered terrible injuries. Cut to Shingo having mostly recovered and happily chowing down on melons.
Gongenzaka claims that he will duel alongside Yuya and Yuzu against the LDS students. Cue Sora pointing out that Gongenzaka doesn't go to their duel school and that he'll be taking Gongenzaka's place. Gongenzaka is speechless.
Made even funnier when combined with Episode 10, where Sora just sits by a wall eating his lollipop and lets Gongenzaka Duel Yaiba anyway, simply because he "didn't feel a good matchup from the guy".
Another from Sora. He calls Reiji's mother an old hag as she explains the rules for the school duels, forcing Yuzu to pat his head to calm him down. It's Sora's facial expressions that sell it.
Episode 9: Hokuto's stunned expression when Shouzou stupidly picks a Star-based field.
Even the bit of drama from Episode 10 has some light moments, two of which are given to us by Masami, the Gem-Knight LDS student. The Ship Tease of Yuya and Yuzu is provided by her, and she also manages to put Hokuto into a corner weeping, via being a Deadpan Snarker.
Before the duel starts, Yuzu is embarrassed by her father when he picks Crystal Corridor for the field.
And on the note of putting Hokuto into a corner, Masami does it again in Episode 11, this time with the expression signs piercing Hokuto.
In episode 14, when Yuya defeats Yuzu's dad with an attack that sends him flying, Yuya holds him in his arms as the man dramatically gives last words and pretends to die. Yuya falls for it and gives a Skyward Scream... then Yuzu angrily smacks her dad with her fan.
"*Manly fainting noises.*" Explanation A line from a fansub, which chose to use that text to dub the noises Yuzu's dad made as he pretended to die.
In Episode 15, after Yuya finishing saying how grateful he is to Gongenzaka for always helping him, Yuzu leaves to Ask Sora to teach her Fusion Summoning, though Yuya doesn't know the real reason. The three kids tell him it's because she was mad because he forgot to say he was grateful for her help too. Yuya freaks out over these and tries to backpedal, only to be informed that it was too late.
Several scenes from episodes 16 and 17. To wit:
Yuya's stomach growling at various points and the various reactions to it, i.e. Ayu taking a page from Yuzu's book and yelling at Yuya.
Yuya's mom completely fan-girling over Michio saying that he'll cook Yuya and his friends a meal after the duel ends.
Yuya's reaction to Sora putting chocolate on sardines.
Episode 19 preview: Yuuya's next opponent is Kyuuandou Eita (Q&A-ta) with a quiz focus.
On the bus, Nico is sitting on top of the seat. When the bus stops, he flies forward. It then cuts to a sign that says 'Please Sit on Seats Correctly,' showing the stick figure sitting exactly how he was.
Yuya's utter failure to answer any of the quizzes.