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.
One from the first ending: In one scene, Yuya, Yuzu and Gongenzaka are hitting a melon while blindfolded. In the next, they're eating the melon, and Yuya has a massive bump on his head, while Yuzu's looking quite sheepish.
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 Shuzou's summoning lessons, the highlight being Sora interrupting it by eating two donuts, comparing it to Fusion Summoning. Shuzou 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 Shuzou 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.
The fact that Shuzou picks another fitting Action Field for both opponents again. For the last one, he mutters even Yaiba's name and thinks about a fitting field, not realizing that he gives another field advantage to the LDS students.
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.
Michio realizes that he is going to lose and accepts it like a man... then he is comically smashed by a giant hammer.
Yuya's reaction to Sora putting chocolate on sardines.
Episode 19: Yuya'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. Most notably the 300 kg question.
Gongenzaka asking Yaiba for assistance. Except, when he first shows up, he looks like he's going to throw Yaiba (who's half his size) through the damn wall, what with the flames and everything. Yaiba understandably looks like he's about to soil himself.
This was most likely not intended to be funny, but out of all the ways Shun Kurosaki could have dramatically come out to confront Masumi, he chooses to do so by running forward and ramming Yuzu out of the way with his Duel Disk arm.
In a similar vein as above, Yuto's solution of punching Shun in the gut to knock him out after interrupting Shun's confrontation with Masumi, while informing him that Yuzu is not Ruri.
Masumi's reminiscences of Professor Marco are ridiculously shoujo and filled with roses and sparkles, making it hilarious.
En (His corgi) waking Yuya up by sitting on his face. Or more like landing on his face.
Yuya being disturbed by all the macabre, occult stuff.
Sora's face when he yells at Yuzu to get to his final duel to watch it.
Mieru's imaginations of her fated person and her Nose Bleed reactions.
Yuya's terrible unluck. He oversleeps, he hits his head under his desk, he trips and falls down the stairs, is accused as an pervert, gets wrong coin side, has a dead opening hand, is glared by thirteen black cats, he trips without any reasons, he draws the Action Trap "Terrible Unluck", and his two strongest monster are switched to defense position.
Although by hitting his head on his desk he accidentally put the Fusion card in his deck which was what he needed to win the duel.
Sora is in an action duel where to describe whatever action they do, the players sing.
After Yuya beats Mieru, she becomes smitten with him, calling him her fated one. Yuzureacts as expected.
Even funnier when Mieru interrupts a moment between Yuya and Yuzu, the background music screeches to a halt.
Or after Mieru asks when will she meet his parents and how many kids he wants. Yuya screams he'll change fate.
This is made more hilarious when it's show all of this happens while Yuzu lands a hard smack of her harisen on Yuya's head...while Mieru is still snuggling him.
Yuya mock strangling Sora after the latter wants to throw a celebration party for qualifying.
When looking for everyone, Yuya for some reason, felt the need to look under tables and in the fridge.