UPDATE!!! RIN’S OTHER HALF IS SOUSUKE AFTER ALL!!!
found this interview snippet on twitter and…UHHH
Interviewer: As for Haruka and Rin, after doing that together and being next to each other, as well as committing to the same thoughts of swimming, you would think that they are two complements of each other, but what are your thoughts?
Utsumi: Well…I would not say that Haruka and Rin are halves of each other. If anything, I think that Makoto would be the other half of Haruka. As for Rin, it’s clear that the person who understands him the most is Sousuke, and High☆Speed has iterated time and again that Sousuke is Rin’s other self. For Haruka and Rin, although they definitely complement each other, if it wasn’t for the common interest of swimming that they both have, they probably would not have become friends because of their opposite personalities.