Warning: My native language is not english, I write this in english to be able to know your opinions (no matter from where you are, is easy to translate from english -i think- so TELL ME WHAT YOU THINK! and sorry for errors and misspelled words).
First: I want to tell you how much I love Kisumi and Hayato. They're the cutest brothers.
Kisumi appears one chapter and everything changes. Sousuke, Haru, Mako, EVERYTHING and EVERYONE. He help us to know about Sousuke's injury. He and his little brother (yes, that strawberry pink hair cutie pie who makes us all go awww when he strugle in the water but is saved by Mako-mom) help Mako to discover what he want to do in the future. He show us jealous Haru (best Haru btw, even more than Happy Haru). He made us all go crazy with that "Kiss me" gossip between Nagisa and Rei. And give us that wonderful english-pronunciation-classes moment between Rin and Rei ("Perfect body").
*lets his anthem starts to sound* (Trey Songz ft. Nicki Minaj- Bottoms Up -aka. Mr. Steal Yo Girl-)
Theory and pictures under the cut
We know Haru. He talks almost nothing and only thinks about water, swimming free style and buying a new swimsuit. And that's a clue: Haru doesn't like change.
He doesn't even change the style of his swimsuits. He doesn't want to think about the future. He gets too stressed about change. He probably likes mackerel that much because is afraid of trying something new and strange that comes from the water. He likes being confortable.
Water makes him confortable: GREAT. Mako makes him confortable: GREAT. Rin makes him comfortable: GREAT. Except when that litle shit decides to move to another country and cry about his daddy issues and let him behind with a thing he doesn't loved before but now he do (swimming with his friends, because that happen thanks to Rin - btw, Rin I love you, sorry about that little shit thing just now).
Haru doesn't bother in having new friends. He is just fine by himself. But Mako is too sweet, no one can say no to that beautiful man and his smile and his need to make everyone happy. But Mako don't excites Haru (perhaps he does in other areas but that's not the point here, people!). He doesn't make Haru leave his comfort zone. Rin pushes Haru. All the time.
It is beacause of Rin that Haru understand he wants to swim (yeah, Mako helps too, everyone did, but the final push was from Rin). They share (even if Haru doesn't know because he is too slow, poor baby) the same dream and motivation. Rin motivates people (Sousuke too, although that did not end well). Haru and Rin share their motivation. They push each other.
Mako, even if he is the mother of everyone, and makes Haru join the Iwatobi Swim Club, he could not convince him to form a swim team in high school. Rin had to move back to convince him, because, once again, THEY PUSH EACH OTHER. such good friends.
So, Rin goes to australia, cries a lot, doens't get a gilfriend (late bloomer say the australian parents but we know better, right, Sou? wink) blahblah. And Haru found himself alone and sad.
Yes, Mako is there, but Mako is probably popular and loved by everyone in their class and grade because is too good to be true. He probably stay and help the teachers and sometimes helps his classmates to study. He probably do good in all sports. And he met Kisumi, another refreshing, good looking-good in sports guy and they become friends. Good friends. (Not enough to stop Mako of chasing Haru because he is too in love to do that. Sorry, because they are neighbors and best friend and their parents are friends too.)
Kisumi probably invites Mako to play and Mako wants to go but doesn't want to leave Haru behind, so Kisumi invites Haru too, but Haru sucks outside the water and he just see how Kisumi and Mako are more compatible. And Kisumi, funny and refreshing as he is, tease Haru about it. Because before Kisumi could leave Haru and Rin doing their swiming things and take Mako to the basket court without too much problem, but now Haru is all alone and sad and Kisumi knows that. Even if Haru doesn't say a thing, he does not want to be left behind by Mako (like Rin did. -Rin, stop breaking everyone's heart already-)
My explanation to Haru's flashback:
Mako and Kisumi are on the same team because they are good players, however, Haru isn't and is on the other team. Mako smiles to Haru, but Haru see how Kisumi and Mako do good plays together.
Then the PE teacher tells Haru to practice more (poor thing, he doesn't even run fast, as we already know) so Mako and Kisumi offered to teach him to play. And Haru is not happy with the strawberry boy who seems to be everywhere and just want to leave and get in the pool (or tub) to be alone. Notice Haru's I-HATE-YOU-eyes.
Kisumi is too friendly and touchy with Haru even when Mako is not around (he is probably that way with everyone, same as Mako-chan). He know that Haru is alone now so he eats lunch with him because they are in the same class. and Haru just goes with the flow (get it? the flow? well, not funny but suitable) and everyone in the class is so confuse to see little quiet Haru with a new friend now that Rin is not there (and the scene is too gay, lets face it).
But, once again, Mako and Kisumi are friendly and good in sports so they go to play together. The basket team probably always invites Mako to play with them. Kisumi knows how good Mako is in basket and probably wants to use his potencial to improve the basket team and always tells Mako to join them, but Mako only accepts to play once in a while without joining the team because he loves swimming (and Haru). Haru just stares with sadness at how his boy is stolen by another boy, who is way too good-looking and he (Kisumi) knows it.
Kisumi just tease Haru a little because he is good-hearted and have lunch with the boy, but he also is a kid who likes to play and knows how much pissed Haru is about Mako leaving like Rin did.
"Oh yeah, I remember I don't like strawberry flavored things, only mackerel. And I hate him. And I hate you because I didn't know you were working. And hanging with him and his little brother who probably loves you already because all children do, you sweetest thing. I won't go to see you flirt at work."
Of course Haru goes to Mako's work even if he says he wouldn't. He went TWICE (Haru, is very low to use your boyfriend's siblings. Don't say Kisumi did the same, that was a coincidence).
"Don't touch me, I came to take my boyfriend to our home. Don't even try it, He doesn't like tall boys like you. and I don't like you. And Rin is back, so gtfo. But don't go with Rin. Or go and annoy Sousuke who clearly is the jealous-type."
"Hello, I'm Shigino Hayato, the sweetest cutie-little-strawberry-pie in the whole world. I will become a pro swimmer when I grow up and will be the capitan of the swimming club in high school. and will marry Coach Tachibana".
Mako-chan is happy to see his friends gathered, is happy to help Hayato and discover that he loves to teach and can put together his love for swimming with his love for (being the mother of everyone) teaching and helping other people.
"Bitch, leave me and my man alone. your purple eyes and pink hair got nothing on me"
Now they even hang together. Kisumi probably invites Mako to have dinner at his house and Hayato is soooo happy of seeing Coach Tachibana and Mako-chan invite Hayato to play at his house with his siblings and they all play videogames, basket and probably go together to the park while Haru appears in the park (where he never goes because it has no pool) to prove Mako's siblings likes him more than Kisumi.
<3
That's my theory on why Haru hates Kisumi. Tell me what you think. BTW, I'm new on everything Free! related, so I haven't read High Speed yet. This is all speculation. Hope you enjoy.
