All Love Is Unrequited: Many of the love interests or love triangles end up like this to a degree. Chizuru develops a crush on Masaki, who's oblivious and has a crush on Shun, who's also oblivious and gets his own brief love interest later into the series. Hisako has feelings for Kaname, who in turn has feelings for Hisako's older sister. Yuki seems interested in the cafeteria worker Kayo but their relationship doesn't go beyond one-sided mild interest, and Yuta gets a girlfriend named Takahashi in Chapter 9, but they break up because she felt she was pushing her feelings onto him (even though he was beginning to like her back at the time). Subverted in that Masaki and Kaname both begin to show signs of reciprocation towards Chizuru and Hisako (respectively), and Yuta and Takahashi have been recently Ship Teased.
Almost Kiss: In Chapter 61, Kaname and Hisako nearly share an almost kiss when Shizuna walks in on them.
Don't You Dare Pity Me!: Due to Masaki's petite stature, she is prideful and hates being offered help, taking it as being looked down on. This changes after Shun goes out of his way to help her, while others would merely give up after getting rebuffed.
This is the theme of Shun's and Yuta's crossdressing cafe during the cultural festival.
Played with when Chizuru views Kayo as a Cinderella-figure as she's a pretty girl working as a cafeteria worker. When two students give her trouble and ask for extra food (due to Yuki getting extra food because he got a wrong order), Chizuru assumes that their Cinderella is a "fragile girl who never complains even when she's being bullied" and attempts to be her Knight In Shining Armour. Before he gets the chance to, it turns out that Kayo is able to handle herself rather coolly, surprising the gang.
Generation Xerox: When the gang visits their old preschool, a boy there has a crush on one of his classmates and gets jealous of Kaname's interactions with her, mirroring Kaname's precocious crush on Kaori-sensei and getting jealous of Koichi's interactions with her.
Holding Hands: Masaki holds Shun's hand while he walks her home, but the moment sours when Shun suggests that they must look like siblings after a woman notices that they're "getting along well." During the haunted house in the cultural festival, Masaki thinks that she grabbed onto Shun's hand after getting scared, but it turns out to be Chizuru's.
Hypocritical Humor: The "Chocobanana" guy making fun of Chizuru's height during the cultural festival has elements of this when he's revealed to be Akira.
Takahashi ultimately breaks up with Yuta because she didn't want to force him to keep dating her when he didn't feel the same way, though he was showing small signs of reciprocation towards of the end of their relationship.
Chizuru often goes out of his way to help Masaki whenever it concerns Shun, in spite of his own happiness. Notable examples include retrieving Masaki's lost bear key chain that she received from Shun, cheering her up whenever she feels embarassed or feels that Shun got the wrong idea about something, and when he used his own Christmas present (a ribbon for Masaki) to replace the messed up ribbon on Masaki's present for Shun. Subverted in that this doesn't stop Chizuru from romantically pursing Masaki.
Imagine Spot: Ryunosuke has this in Episode 8 involving different scenarios between Yuta and the gang (e.g. Super Sentai, samurai, delinquent fights).
In-Series Nickname/The Nicknamer: Chizuru gives everyone nicknames: Kaname as "Kaname-chi," Shun as "Shun-chan," Yuki as "Yukki," Yuta as "Yutan," and Masaki as "Mary" after "Mary had a Little Lamb." Kaname also calls Chizuru as "Monkey Boy."
Love Hurts: In Chapter 26, Masaki attempts to give Valentine's Day chocolates to Shun, but sees him getting confessed to by another girl, so she runs off upset. The next chapter resolves things with Chizuru and Masaki eavesdropping on Shun later politely declining said girl's confession.
Love Interest: Each of the guys get their own (e.g. Hisako's sister and Hisako for Kaname, a girl who Shun meets during their class trip for Shun, Kayo for Yuki, and Takahashi for Yuta) or even a love triangle (e.g. Shun/Masaki/Chizuru and Hisako/Kaname/Shizuna).
Sibling Triangle: Hisako has feelings for Kaname, who in turn has feelings for Hisako's sister.
Meaningful Echo: In Chapter 18, where it turns out both Fuyuki and Mamiya are both nervous about what they're supposed to tell their parents about the other.
Fuyuki: [earlier in the chapter] If she calls the house when I'm in the shower and my parents... My parents end up answering it... If, if that happens, and they ask me, who was that just now? H-How... am I supposed to answer...
Mamiya: [later in the chapter] I mean, if you walk me home... What if my parents see you and ask, "Who was that just now?" ... How am I supposed to answer that...?
Fuyuki:Ah... Who knew it'd be so embarrassing when two parties like each other. ... Just say I'm your boyfriend.
Meet Cute: Shun with his love interest in Chapter 43 and again in Chapter 46.
Memento MacGuffin: Kaname carefully keeps a photo of Hisako, Hisako's sister Shizuna, and himself during a barbecue. This hints at his feelings for Hisako's sister (to Hisako's personal annoyance). Masaki strongly cherishes a bear key chain given to her by Shun at a summer festival.
Mistaken for Aliens: When Akira first meets Koichi, he mistakes the latter for this after watching a television show with a similar premise.
Moment Killer: Hisako's older sister Shizuna has a particular when she walks in on Kaname's and Hisako's Almost Kiss in Chapter 61.
No Name Given: Hisako's sister's name is not given in the manga and is only called "Oneesan." The anime adaptation gives her a name: Shizuna. Also, Takahashi is only known by her last name in both the manga and anime; the anime credits reveal her first name, Rina.
Twice in Chapter 20 between Kaname and Masaki. Two of Masaki's classmates see Kaname and Masaki sharing an umbrella together and mistakenly think they kissed. This only furthers when Masaki later purchases a gift for Shu and two guys start teasing Kaname because he's holding it (so they're under the impression that it's for him)
Kaname also suffers from this in the same chapter with Chizuru, who, thinking that Kaname and Masaki are dating and kissed, tries to take Masaki's indirect kiss through Kaname. Koichi bumps into them and assumes that Kaname and Chizuru must be the ones dating.
Yuki: Come on, she asked you something. "What's more important, me or your work?"
Chizuru: Don't make a girl cry!
Kaname: She's not asking me that! It's not like a fight between a man and a woman or something!
Yuta: Kaname, no matter how good you are at studying, you can't even understand a woman's heart.
Kaname: What's this about a woman's heart!
Older than They Look: In his high school years (and even present day where he's nearing his thirties), Akira gets this reaction.
One Degree of Separation: Kaname, Shun, Yuki, and Yuta have known each other since preschool, their high school teacher Koichi and his best friend Akira helped teach the gang's preschool class on an occasion when they were in high school, and Yuki played with Chizuru a few times as children.
Pinky Swear: In a flashback, Akira promises Koichi that he'll do his best (and subsequently pass the school year) on the test in this fashion. He doesn't pass the test, but manages to pass the school year after crying to his teacher about having to "cut his fingers off," taking the pinky swear seriously.
She Is Not My Girlfriend: Hilariously averted in Chapter 16. When a foreign woman is convinced that Chizuru is her son Michael and tries to bring him back home to America with her, she notices Shun and mistakes him as Chizuru's girlfriend. Chizuru goes along with it, probably thinking that it would help the situation, but it only makes things worse when the woman decides to bring both of them with her, leaving them to beg for help from the rest of the gang.
Twice Shy: Shun's younger brother Fuyuki with his girlfriend Mamiya.
Umbrella of Togetherness: Played with in Chapter 20. Kaname offers to share his umbrella with Masaki after she loses hers, and they are mistaken for a couple by two girls. Chizuru and Masaki later bump into each other while shopping and it starts to rain, but they don't have umbrellas. A shopkeeper assumes they're a couple and offers them an umbrella. Masaki, still rattled from her earlier mishap and not wanting Shun to get the wrong idea again, refuses and runs off. The next day, Chizuru helps Masaki with her present for Shun, and a grateful Masaki offers to share her umbrella with him, to Chizuru's delight.