I can see why you're so taken with him.

I just thought that I'd spend my last year as a high school swimmer in my hometown, enjoying it my way.

Sousuke is a very quiet boy with a hard to crack exterior, but when you look past that he's actually a very thoughtful person who's really dorky. He's very quiet and keeps to himself. He'll carry on a conversation just so he isn't downright rude or disrespectful to the person he's talking to, but, he's not really the type of person who goes around starting conversations. In fact he's the type of a person who will just stand by quietly in the background, and will only speak when it's necessary or if he's asked a question. He's also really stoic looking and it could be said that he has permanent resting bitch face syndrome. So that coupled with his quiet nature is probably why he doesn't talk as much as the others on his team do.

It also seems like he needs to warm up to people before he's buddy buddy with them. As seen with how he treats Nitori and Momotarou at the beginning of the series. When he first met them he didn't really say or do much, other than when he introduced himself to Nitori. At first he didn't really interact with them much, but as time passed on and he gradually warmed up to them. As seen where he takes Nitori under his wing and helps train him. He also jokingly told Momotarou that he was going to release the stag beetle the other had caught and given to Rin for his little sister. He's extremely loyal (probably one of the most loyal characters in this series) though and is definitely the type to look after his friends. He's also extremely protective of his friends, and will intervene wherever he sees fit. Which is why he confronted Haruka early on.

Like most boys, Sousuke's determination knows no bounds, especially when it comes to things he's very passionate about. He's the type of person who will commit to whatever he's doing and see it through to the end. When he sets his sights on something he usually goes in for the kill. Even if it means fucking himself over in the process. Once he's set his mind on something he usually doesn't go back on his word. But when he has to come to terms with things he takes his time considering the possibilities. And when he does come to terms with the possibility that he has to give up on something he loves, he accepts it with grace. This is why he decided to go back to Samezuka instead of just quitting or giving up on his dream. One, he wanted to make sure that his rival/friend was going to fare all right on his own. And two, wanted to be able to get the chance to swim with his friend one last time before he had to give up and abandon his dreams so quickly. This was one of the reason why he lied to Rin. Because he's usually pretty straightforward, and isn't one to really beat around the bush. But he did this because he didn't want the other worrying over something and not focusing on achieving his own goals. He also lied because he knows how emotional Rin could get, he didn't want his friend to cry over something that he did to himself because of his own thoughtlessness.

But in order to truly understand Sousuke Yamazaki, you need to understand how serious he is when it comes to swimming. Swimming is like his life, and he doesn't take it for granted; or see it as a something to do with your friends to pass the time on a hot summer day. He's poured his entire being into the sport just so he can reach his goals of becoming a professional swimmer and getting the chance to swim with and against his rival and childhood friend, Rin Matsuoka. So when Rin went abroad to Australia to better his swimming career, Sousuke took that as an initiative to better his own skills as well. He would feel horrible if he wasn't able to provide much of a challenge to Rin when he got back home to Iwatobi.

So that's why he threw himself so aggressively and headstrong into training. That's why he didn't really pay much attention to how his body was reacting to his rigorous training until his shoulder had completely shut down on him. And by that point it was too late to go back and fix anything. This is one of the main reasons why he decides to help coach Nitori and help him improve his time. He doesn't want to see someone else making the same mistakes that he did. Especially when the share the same goals of getting to swim with Rin. He's not bitter about it either, like most athletes are when they can't carry on with their dream. He recognizes his own mistakes, learns from and moves on with his life. He has his moments, like when he first initially gave Nitori his blunt advice to get better. But he was able to recognize where Nitori was going to go with it, stepped in, and intervened before Nitori could mess up his career like Sousuke did with his own.

Because he takes the sport so seriously, it's understandable why he has such a negative confrontation with Haruka Nanase. A boy who is as gifted as Haru and only wanting to swim "free" is kind of a joke to Sousuke. He doesn't know Haru well, only that Rin completely idolizes his swimming. And yeah, he admits that Haru is an excellent swimmer, but it's hard for Sousuke to wrap his mind around the effort Haru puts in to swimming. It would be something if he were like a lazy genius, but it isn't. It's completely frustrating to Sousuke because it's obvious to him that Haru doesn't really care about swimming as much as he and Rin do. And as any good best friend/rival, Sousuke wants to see Rin excel in his swimming career. He wants the best for Rin, and he really felt (at the time) that Haru was dragging him down. That's why he confronts Haru and tells him to leave Rin alone, because he doesn't want to see his friend blow his chances at making it big or getting scouted. Unfortunately this confrontation was before he truly knew what happened and that Haru and the other boys on the Iwatobi team actually helped pull Rin out of the slump he had been in last year.

His friendship with Rin is... actually quite an odd one. He spent most of his childhood as Rin's best friend, but he never really considered himself one? Even if he... basically did everything a best friend would do. He visited Rin when he was sick and cheered him up, they would get into stupid pretend sword fights with broomsticks when they were supposed to be cleaning the classroom up, and they pushed each other to do their very best in swimming. They even have the most dorkiest handshake greeting ever. Sousuke knows Rin the most and understands how he works, but he never really considered himself as one of Rin's friends. He probably took that childish fight they had to heart and thought that since he kind of drove Rin away after they fought over the relay that they lost, that he could at least be Rin's rival instead. It's only when he learns what happened to Rin last year and how much the Iwatobi boys helped him out makes him realize that he wants to be included as one of Rin's friends as well. He deeply cares about Rin and at the end of the day he wants nothing more but to see his friend accomplish his dreams.

You complain, but you'll do it for them, right?

Sousuke is kind of lacking in magical/super powers. But he kind of makes up for that with his swimming. He's a very gifted swimmer and really excels in anything that has to do with the butterfly stroke.

THE TIME WILL COME, WHEN YOU'LL HAVE TO RISE ABOVE THE REST.