The Swimmer is a friend of Ruby's.
He debuts in Chapter 206.The swimmer wants to capture a Feebas in order to sell it and get rich. He is foiled by Ruby, who gets tangled up in the fishing line, so the swimmer ropes Ruby into helping him fish.
When he sees Ruby has caught one, he lunges to grab it, but this startles Ruby and causes him to drop Feebas into the water. They keep trying but they can't find it again. The swimmer is depressed that it got away.
He and Ruby accidentally catch the attention of a feuding Zangoose and Seviper. They are saved by Feebas and flee into the Weather Institute, where the swimmer tells Ruby off and (rightfully) gives Feebas all the credit for their survival. He adds that Ruby should accept Ruby as one of his Pokémon. (Inwardly he plots to exhort Ruby to breed the Feebas and give them to the swimmer, making him even richer.)
He follows Ruby onto the Weather Institute roof, arriving in time to see Norman hurl Ruby and his Marshtomp into the wall. As Ruby prepares to fight Norman, the swimmer tells him to just apologize. Ruby refuses and requests that the swimmer not watch the upcoming battle, but the swimmer ignores his request. He meets up with Maryann and Tyler, who are passing by, and watches the battle with them.
Afterwards, when Norman gives Ruby his permission to participate in Hoenn's contests, the swimmer is touched by the father-son moment between them. He helps load Ruby's unconscious body into Maryann and Tyler's van and sees them off. After they leave, the swimmer realizes he never got a chance to ask Ruby about breeding Feebas for him.
The swimmer apparently goes to Slateport City, figuring that Ruby would turn up to challenge the contest and the swimmer could convince him to breed Feebas. He is horrified by Wallace harsh treatment of Ruby and Wallace's hesitation to try to save him. The swimmer sends out his Gyarados and saves Ruby himself, handing him over to Wallace.
The Swimmer seems to be selfish, trying to steal feebas from Ruby, but is actually deep down nice. This is seen when he is defending Ruby from Wallace.