WASHINGTON — The tail of the whale is dragging on the convention center floor, and everyone is looking.

It is attached to a full-body costume made of blue felt that was carefully measured and velcroed to fit its owner, a 12-year-old boy named Theo Chevis. He is hoping to get years of use out of the whale costume, so he ordered it to be a little big. He doesn’t mind the way it drags; he has a tournament to think about.

The costume actually is not a whale at all. It’s a Kyogre, a whale-like character from Pokemon. One of the strongest Pokemon characters to exist, he says. “A true behemoth, really.”

The fuzzy fabric took up more than half of Theo’s suitcase when he and his dad flew from London to Washington for the Pokemon World Championship, taking place at the Washington Convention Center through Sunday. On Friday morning, Theo’s dad left him to his own devices in the world he loves. As he makes his way to the concession stand, Pokemon players stop their card trading and point to Theo, the 5-foot Kyogre.

This was the day Theo had been waiting for ever since he ordered his custom-made costume online in the spring, without his mom’s permission. More than 2,000 Pokemon fans, gathered in one place for the most important tournament of their year.

There are the Pokemon trading cards, which debuted in 1996 and are still seen as the core of true Pokemon fandom. And now almost equally important is the Pokemon video game, the reason Theo is here, hoping for a tournament berth. He didn’t qualify in advance, so his one remaining chance is an afternoon contest in which 75 kids will try for four spots in the video game championship for Theo’s age group.

He is about two hours away from the start of that tournament when he makes it to the concession stand. He pulls out the American money he is keeping tucked into his registration form and, with no parent watching, buys barbecue chips and a Mountain Dew.

“Thank you,” he says in his 12-year-old British accent. He says it again a few minutes later when someone yells, “Woah!”

“Can I take a picture?” asks the admirer, who looks college-aged. “All right,” Theo says. The fins of the Kyogre are sewn on like butterfly wings, so he spreads his arms wide and looks at the phone’s camera. “No, with you man!” the guy says, then kneels down in front of Theo to take a selfie.

Theo is one of the only ones in full costume, which makes him stand out even among the throngs of people in Pikachu hats. The attendees come from 30 countries and represent every age between 6 and 60, according to one tournament organizer. “These are the most incredible people,” Theo says, entering the game hall where a giant inflatable Pikachu is hanging from the ceiling. But he is a Kyogre, and he is ready.