While the end result of Tenma's Love Letter to Karasuma is Played for Laughs (a really long scroll where she forgot to write her name), the fact he actually decides to stay at school in Japan for another year because of it is this. Especially when he's shown rereading the "Please, please don't go!" parts.
When Yakumo is feeling a bit down after going through a hectic day at school (including her first encounter with Hanai), Tenma suggests doing something different for dinner. Yakumo reads her mind and hears Tenma thinking, "I don't know what's wrong, but I'll cheer her up!" A good big sister right there.
Karasuma offering Tenma his kappa equipment when it's raining because he notices that she's sad, assumes it's because she's worried about getting home safely and and also expresses his concern in leaving her behind helpless. He interrupts her attempted love confession in the process but it doesn't stop her from smiling and accepting his help.
In episode 15 after Mikoto rips her dress chasing after Eri, she goes to Hanai's dojo to change clothes. She tells him she tripped but he doesn't believe her. When Harima shows up to try and return Mikoto's jacket and purse Hanai misunderstands and think Harima did something to Mikoto. The sheer look of surprise on his face followed by the rather vicious way he challenges Harima to a fight because he even thinks he attacked his friend is particularly touching and shows how much he cares for her.
Eri and Harima dancing together at the closing of the Sports Festival. This is not long after Harima gives the injured Eri his jacket and vows to win the men's relay race in her honour - and he succeeds, even though it meant revealing the bald head he feels ashamed of to do it.
Harima and Tenma's "date," especially the end when the latter is grateful for the former's help and gives a Sleep Cute moment. Harima actually helping her choose a present for Karasuma despite his feelings and resentment tops it off.
From the season 1 finale.
Tenma: Am I dreaming?
Karasuma: We all are. We are in a dream called youth. Someday we'll wake up from it. But the memories will stay with us forever.
Hanai getting Yuuki home because the latter's glasses broke, and the scene with the sunset where he helps her see it through a broken shard of her glasses so she doesn't miss out.
Karasuma thanking Tenma for her good intentions (such as trying to celebrate his birthday) at the school festival. Topped in the anime where he actually smiles for her too, right after she expressed her wish in seeing his smile like in her photo of him.
Imadori watching Ichijou sing in her band performance, which moves him enough to cancel his date - this is in contrast to him normally trying to ditch his time with Ichijou by picking up nearby girls.
In the ♮ (natural) manga chapter "Ballad in Blue" in volume 7, during a meteor shower, Yakumo reads some of the sleeping Tenma's wishes. After a few silly ones (such as wishing for maths and English to become extinct), she reads the last one which says, "I want to become a reliable older sister and to be able to protect Yakumo."
Everyone celebrating it with her and having fun together is touching as it is.
Yakumo remembers Harima's birthday unlike everyone else, who is too busy celebrating Tenma's, and gives him a present.
And on the same night there's Harima covering for Karasuma at work while the latter makes a last-minute visit to the Tsukamoto's to see Tenma.
Harima mistakenly writing his and Eri's names on a love charm at the Kyoto trip. The look on her face when she finds the charm is priceless.
Akira's story about the village girl (Yakumo) and the hunter (Harima).
Nishimoto comforting Mai when she thought her father was lost at sea.
Harima and Eri sharing a blanket when they took shelter in a temple.
Yakumo sending Tenma to help with Harima's manga in her place so that she can help Harima out and give them some alone time.
Imadori actually becoming concerned for Ichijou's safety when she starts hanging out with a dodgy older work friend and going out to rescue her when she's brought into a shady club with the friend and feels uncomfortable about it.
Though it can count as a tearjerker too, there's Yakumo slapping Eri when she catches her having a Jerk Ass moment with the stressed-out Tenma whom she's been purposely ignoring due to misunderstanding her relationship with Harima. More proof that Yakumo will not stand by and let someone belittle her older sister.
In Chapter 212, there's Tenma's attempt to write an apology to Eri on the snow, and Eri's smile at the end of the chapter.
Tenma, as a child, encouraging a sakura tree to bloom one last time before it gets chopped down.
When it's finally revealed that Karasuma really did love Tenma. The way he describes his love for Tenma is very endearing.
Karasuma: One year. I asked God...I asked God to let me be free for just one year. Somehow, some way...I wanted to live my life without anymore regrets. One year. I decided that after that year, I would lose all those thoughts and ideas...But I promised myself one thing...I promised that I wouldn't fall in love with you. But...I couldn't keep that promise. Just looking. It's what I had decided. But I couldn't do it. Tsukamoto-san...I love you. I love you more than anything else in the world. I love your smile. It brings me the most happiness in life. I love the way you talk. It echoes from the bottom of my heart. I love the way you cry. It makes me want to protect you that much more. I love the way you sulk. It makes me notice my mistakes. There's no one that I'm more connected to in this world. No one. You're a bit careless and sometimes selfish but...with all of that together-I love you. You're completely pure. That's everything I think about you. The one whose touch I want to feel the most. Even if you know I'm hurt, even if you're scared...Please don't hide from me. Please believe in me through it all...It will put me at ease. I love you..I love you...I love you...I love you. Let me touch you. Let me feel your touch. Save me.
When Tenma finally realizes Harima's feelings. She doesn't return them but she acknowledges them and thanks Harima for always helping her and being a good guy in the end.
* Harima and Eri becoming engaged.
Karasuma instinctively protecting Tenma during the pie fight at the reunion.
In School Rumble Z, when Imadori gives Ichijou a ring.
The very last chapter of School Rumble Z, where Yakumo is giving a speech to the graduating class and starts to tear up when she recounts all of the good memories and how much she'll miss them. Tenma rescues her by taking the mic and telling her that she did a good job. Then, Harima wheels the ill Karasuma in and tells him, "Hey, look closely up at the stage. That is the girl both you and me fell in love with. She's a great girl, don't you think?"