In-universe, Mephisto thinks that Rin becoming an exorcist is the funniest thing in ages.
Rin and Bon's dispute over who'd kill Satan... "YOU STOLE MY DREAM!" "NO I DIDN'T, MY DREAM'S ORIGINAL!"
Rin's analysis of Bon in Episode 5.
Rin: "Two tone hair... piercings... he's got that rebellious look...damn he's cool."
Chapter 8: Kuro is going on a rampage and Rin opts for persuading the cat sith to stop since he can read his mind, assuring he will "use his head". Cue Rin stopping Kurowith a headbutt.
Mephisto's Imp Oatmeal and its effects on Rin.
Rin: "I think I see grandma..."
Yukio: "Grandma...? Wait! Get yourself together, brother, even though we've never met our grandma!!"
In the Filler episode The Phantom Chef, there is the running gag of Rin reaching for his sword for various reasons; the food is ridiculously expensive in the cafeteria, Mephisto isn't giving them a proper allowance and finally when he finds out that Yukio makes more money than him. When he tries to take it out on Mephisto, the headmaster reasons that he likes a certain bill because it has a face of a famous person, or such.
2000 Yen bills are considered really rare in Japan (tourists, on the other hand, can often get them when they change bills outside Japan).
Not to mention him going on about how the students need to experience culture and expensive food... then eats a ramen cup.
Mephisto's reaction when he finds out that Amaimon managed to get a certain "rare" yukata, with all these pictures of cute little girls printed all over it.
Not to mention the fact that when Amaimon was given the chance to go anywhere in Assiah, his first reaction is to act like a typical tourist on a vist to Japan. Judging from the amount of stuff he was able to buy Mephisto must have not only expected this sort of behavior from his little brother but had given him a much bigger allowance then what he gave Yukio and Rin when they asked for money. Apparently being able to get plenty of souviners is more important to Mephisto then the ablilty to pay for basic necessities like food.
The cooking duel between Rin and Mephisto's Ukobach familiar and the resulting conversation.
Rin and Ukobach giving Yukio a bento stack at the end.
Yukio: "Hurghhh! I hate lunch!"
Amaimon sheathing and unsheathing Rin's sword over and over, changing his appearance along with it.
Mephisto after receiving the student's Christmas/Birthday cake:
Mephisto: "What? Giving a cake that celebrates the birth of God's son to a Demon? What are they thin-... WOW!!! FANTASTIC!!!!"
Earlier in the same episode, Rin brings in the cake. It had already been established that Rin and Yukio were born on December 27th, so they had always celebrated Christmas and their birthday together. When Rin brings the completed cake, decorated Christmas style, neither he nor Yukio think that there's a problem with the cake, much to the other students' exasperation.
Rin: "Oh, so Christmas cakes and birthday cakes are totally different..."
Yukio: "Even at this time, we're learning so much, right, brother?"
Shiemi tries to conduct a meeting to plan for the party, only to confess that she doesn't know how a birthday party works. Rin's reaction is to skyrocket about the room in a puff of smoke.
The party was a surprise for Izumo, and Shima was assigned the task of getting the presents. Since he doesn't know what Izumo would like, he asks Paku for help. Izumo sees them talking together, thinks they're seeing each other, and decides to spy on their "date" to the mall. Hilarity Ensues.
Doubles as a Heartwarming Moment, at the end of the Impure King Arc, Rin's True Companions, save Sheimi, spell out "SATAN" for a photo with their bodies.
Furthermore, Juuzou formally proposing to Mamushi by asking for her dad's permission.
Shura explaining to the class that Rin is "allergic to holy water."
Bon: "Allergic to holy water? I've never heard of that!"
This bit from the dubbed version of Episode 20 of the anime as Yukio corners Neuhaus.
Yukio: That spider web on your shoulder, spun by demonic powers. The monastery's covered in it so there's no doubt that you're involved.
Chapter 40 has the cram students begin to deal with "The Seven Mysteries" of True Cross Academy, i.e. demons. The first one up is a Wedding Kimono Ghost... who turns out to be a Transvestite. And wants to kiss a guy to find his/her Mr. Right. Hilarity Ensues.
How Rin gets past security blocks: With a serious face like he belongs there.
Rin: "Rin Okumura: Exwire"
Followed by his "reason" for coming:
Rin:(to Yukio) "I'm worried about your second pair of glasses."
Yukio: "That makes no sense."
Chapter 41. Rin, Yukio, Bon, Shima, Konekomaru and Takara. Crossdressing. Even Shiemi was laughing.
Chapter 42 is actually very serious....except for the background events around the Cram Students and especially Rin. The audience sees them dealing with a giant possessed statue and a possessed train. Or, rather, Rin deal with them....
In chapter 43, Mephisto waltzes in, in his dog form. Izumo remarks that he is cute, and he promptly changes back into his normal form. Izumo's reaction was priceless.
Same chapter, but earlier: Rin casually admits to Godaiin that yep! he's a half demon. Priceless expressions from everyone in the background.
When Yukio is visiting the supply store, he instructs Rin to stay outside and not touch anything. Rin walks up to the garden gate and leans forward. The instant he touches it, there is a brilliant flash and the gate falls over.
Yukio and Rin get in an argument, and Rin breaks Yukio's glasses. Yukio really loses his composure, then is suddenly interrupted by a phone call. After calmly recieving an assignment, he pulls open a drawer containing at least ten pairs of glasses.
From the bonus shorts on the second dvd
Rin getting lectured by Yukio about hiding his tail because of a bunch of close calls involving Shiemi almost noticing, the scene cuts to Rin using his tail to make it seem like something is trying to burst out of his chest.
Mephisto and Amaimon sitting around playing videogames which ends with Amaimon eating Mephisto's custom made game controller.
The English dub outtakes. Everyone in the main seven gets in the act, as well as a large number of supporting characters.
In the movie:
Mephisto is petting Hamster!Amaimon, and talking to Rin and Yukio. Amaimon randomly bites him, and he skips a beat in pain before continuing.
Yukio tries to pop candy into his mouth from the dispenser, only to hit himself in the face with it.
Shura sneaks up on Rin to put him in a headlock, motioning to Shiemi to be quiet.