As a moments page, all spoilers are unmarked, as per policy!
The relationships Takeo has with Suna and Yamato in general. Suna is his best friend since they were five and, though Takeo saw literally every girl he ever liked prefer Suna over him, Suna shot them all down for the simple reason that they would badmouth his best friend behind his back. With Yamato, their interactions are frequently Adorkable and hilarious, and we see how much Takeo cares for Yamato with entire panels being dedicated to his thinking about how much he loves her coming up multiple times.
Yamato and Suna, too. There's no competition between the two, and both encourage each other's relationship with Takeo. Yamato is the first girl to really treat Suna like a person, instead of just falling for his looks.
While all the girls are attracted to Suna, all the guys admire Takeo. When Takeo announces to his male classmates in episode 4 he has a girlfriend, after the initial surprise they're celebrating his success and even tossing him into the air (which ends badly given it's Takeo).
Even better? He wasn't telling them to boast. He was telling them that Rinko had set up a mixer, and Takeo was hoping his buds might find girlfriends of their own. It took some time, but one of them does fall in love with Nanako.
After Takeo rescues two of Rinko's friends from the burning restaurant (the same two who had mocked him behind his back), all of her friends find themselves briefly smitten with him, although they quickly deny it. When Rinko says that she may have explained badly what she liked most about Takeo, the others stop her, saying now that they get it. Rinko tells them that they can't have him, to which they point out that even if they tried, it wouldn't work. It was obvious to all of them that he only had eyes for her. What's more, none of them ever doubt Rinko's or Takeo's love for each other after that point.
Suna insisting that Takeo enjoy his date with Yamato during episodes 8 and 9. This after finding out that Suna's father was going to have surgery on the same day as Yamato's birthday. Takeo does what his friend tells him, but ends up going to the hospital anyway to support Suna, much in the same way Suna had always been there for Takeo through thick and thin. Yamato shows up as well, having been working on a garland of origami cranes for Suna's father for luck.
The conversation between them is particularly heartwarming. Suna blames himself because he was out at the bookstore when the attack happened. Takeo, insists that the only person who feels that way is him. Not his mother, father, sister or best friend. Suna is so touched he sheds one of about a million tears he's holding back and can only muster a barely audible agreement to Takeo.
Ai deserves mention here too, because she runs over to Suna when she arrives at the hospital, squishing his face in her hands in a cute big-sister gesture. She apologizes for the fact that he had to handle this without her and their mother, and asks if he's okay.
Takeo and Yamato finally say "I love you" to each other in episode 11, while watching the sunset on the beach. It's a very sweet moment, especially given the many times Takeo's thought the words in previous episodes.
After Takeo and Yamato get lost in the woods, Yamato's friends think they're "spending the night together" in a very different way, and cover for her when Yamato's parents call to check up on her.
Takeo and Yamato's Surprise Party for Suna in episode 13. He noticed they were being unusually nice, but didn't think much of it. Later he's not sure how to feel about said party, until Takeo tells him to just be happy, causing him to smile.
Later in episode 13, Takeo intentionally gets in Hayato's face, and asks if he can help him in the most menacing tone imaginable. Not because he suspected he was up to anything(he was) but simply because he thought he was picking on Suna.
...and later still, Takeo and Hayato bump into one another in the park, and Takeo apologizes for being rude. They eventually hit it off, and Takeo takes him to his home for a meal and a place to crash for the night. It goes to show what a truly nice guy Takeo is that he'd show such hospitality to a person he at first mistrusted and doesn't really know. Hayato, for his part, is truly thankful to Takeo, and politely apologizes to both his parents for causing them the trouble.
Ai is given some time alone with Takeo at the theme park in episode 14 thanks to Hayato's scheming. She nearly gives him a Love Confession, until they're interrupted by an announcement overhead about the parade starting soon, and then tells him to go find Yamato and not cheat on her.
After blowing his group date with Nanako, Kurihara attempts to redeem himself by grabbing the star from atop a large Christmas tree in the karaoke bar. As he climbs he, he starts to give off reasons why he's doing it and that he likes her. Unbeknownst to him, Nanako came back to the bar, and ends up hearing everything he was too shy to say to her earlier. He then gives her a Love Confession, and the two enter a Relationship Upgrade.
Episode 18 features Yamato singing Happy Birthday to Takeo in English. She also calls him right at the second of midnight. It also, after an initial hiccup on the matter, features their first kiss.
Episode 19:
Takeo's parents, especially his mother, gets A Day in the Limelight. She also gives birth a baby girl. Immediately before the birth, Takeo's mother makes a point of asking for her bag so she can have the safe birth charm that Yamato made her.
When Takeo's mother is in the hospital giving birth, Yamato and Suna come to his place and stay with him so he won't be alone. Yamato stays until she has to head home and Suna stays until Takeo's father gets home.
Episode 24, when Takeo is faced off against a guy who unlike him, knows how to bake sweets much like Yamato. He tells the former that he'd be a much better fit with her and after hearing all his reasons, poor naive Takeo suddenly feels inadequate. Fortunately however, his fear is unfounded, as she only cares about him, and wasn't even remotely interested in the other guy. The other guy also sees what a kind, thoughtful person Takeo is when he rushes towards the bakery in order to retrieve his cooking utensils in time for the competition. Despite everything he had been saying regarding stealing Yamato away, in the end he can't bring himself to break apart a couple so kind and so dedicated to each other.
Takeo giving Yamato a hug from behind after she unexpectedly visits him.
During Takeo's period of self-doubt, he vocally expresses concern that maybe he's not right for Rinko after all. Nanako, the same Nanako who had once talked snidely about him behind his back, tells him that Rinko is a smart girl who knows what she wants, and that she had picked Takeo. All said with absolute sincerity, as Nanako had come to appreciate Takeo's friendship and understand why Rinko liked him to begin with.
Calm, stoic Suna obviously adores Takeo's baby sister Maki. It's clearly mutual, since Maki excitedly squeals and reaches for Suna whenever she sees him.
Takeo and Yamato's Long-Distance Relationship after her father is transferred to Spain is this; even though they're miles apart, their love is just as strong as ever. Further enhanced by all their friends doing their best to help Takeo in their own way; Yukika lends him romantic films and books, while their other friends, including Ichinose, give Takeo words of encouragement, support, or advice
Sunakawa's status as Takeo's best friend really shines here, as he helps Takeo get his passport, takes him to pick vegetables in the highlands when Takeo is feeling particularly down, helps Takeo set up skype so he and Yamato can stay in touch, and helping Takeo study for entrance exams unil Suna himself is falling asleep.
Their reunion in Spain - so the entire first half of the final chapter - is incredibly heartwarming; after hearing that Takeo broke up with Yamato, their friends once again show up to help Takeo get to Spain so he can apologize. What follows is Takeo's journey across Spain, dotted with his exploits helping people out as always, until he makes it to Yamato's town. There, in the midst of a flower festival, he and Yamato make up, declare their immense love for each other, and promise to get married.
