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Sosuke serves the purpose of a parallel to Haruka (quiet demeanor, affection for Rin, exceptional talent), and further insight into Rin's past and character. His arc is a prominent part of the story, second only to Haruka's turmoil over his future.
He is first introduced so you would think he was an antagonist, until Rin greets him fondly with a special handshake. He retains some of this intimidation when he confronts Haruka several times over Rin.
He can also be seen as what Haruka should not become. Driven, to the point of breaking, and unable to use his talent. Sosuke's injury is the nail in his swimming coffin, and Haru's indifference could be the same if he let it be.
Sosuke's blindness to what a team effort brings to swimming also serves to show how strong the bond between the other characters have grown.
Sosuke serves as the main sublot of the season (and for certain episodes, the plot), as well as a foil to Rin (he is much calmer and independent than Rin) and a counterpart to Haruka. His ending is left ambiguous, implying he either moved on or was broken completely.