Yuuji and Shouko, whenever Shouko isn't her Yandere self, are a walking pair of Heartwarming:
During episode 7 of season 1, after a couple makes fun of Shouko's dream to be married to Yuji, she runs off the stage, while Akhisa tries to kick them out. Later, Yuji seems to show indifference to Shouko's feelings, but in reality was waiting for that couple earlier, and decides to have a little "talk" with them. Later he shows up looking a little worse for the wear, and tells Shouko that he thinks she has a good heart, and that her dream isn't bad, just that he's the wrong guy for her. This only convinces her that he is the right guy for her.
Episode 11 of Season 2. The story where it all started between Yuuji and Shouko. Yuuji is the Child Prodigy who was an Insufferable Genius while Shouko is a rich girl who was a Shrinking Violet that just wanted to have friends. Shouko was supposed to be transferred since her parents were under the impression that she isn't making any friends, but she does have her "friend" in the form of Yuuji. Then Yuuji's plan to get his upperclassmen backfired, and it hit Shouko, who was just protecting her only friend. It only took Yuuji to hear her scream "I don't want to be transferred" to trigger his Knight in Shining Armor mode, something that hasn't happened before. He ordered Shouko to run out, thereby severing her connections to the said incident, then he took all responsibility, omitting Shouko from the picture, sacrificing his own future just so that she wouldn't get transferred. His mother was proud of this, saying that she'd be more proud of a son who'd stand up for someone than just being a Child Prodigy. Then Shouko promised her that she would be his bride someday. The last sentence alone was heartwarming enough to summarize the episode.
Episode 8 of Season 2. Back when Minami first transferred to Japan, she had a hard time adjusting to being in Japan (including alienation and language difficulty). Yoshii is the only student who even tries to befriend her. Since she couldn't understand Japanese very well at the time, he memorized a French phrase in order to try and talk to her (though in his typical idiot fashion, Minami's native language is German). She finally decides to look up what Yoshi has been trying to say to her all this time. Since he's speaking in French, she's forced to translate it into English, and then German. After a large amount of effort, she finally figures out that he was saying "Could you become my friend?"