"Well... You can go faster when you're not having to dodge stuff. Like over water! Running so fast you don't sink's really impressive."

"Yes, I am fastest over water, though I could go slower than this and not sink. Perhaps there is a great circle of the planet that is entirely water!" The blur around them becomes mostly blue and starts to feature fewer sharp swerves. 

"Yeah! That'd be cool, if you could run around the whole planet without leaving the water."

"It appears there is no such great circle. Too many inconvenient islands. But if I follow a hyperbola instead . . . Ha!" Now their path is all blue.

"That's great! Have fun figuring out paths!"

"I certainly will!"

"See you later then!" And off to the third blur?

As a matter of fact, no. Entei goes back to that bit of forest and stops dead still in the exact same spot he was previously standing.

Someone else is now standing right in front of him. 

"Hello. I see you have ceased being destructive."

"Uh, hello!" Nereus says.

"I had been chasing my brothers in an attempt to calm them down, but I am the slowest of the three of us, and was avoiding cities besides. I thank you for your help. In return, would you and your companion like to be returned to your home?"

...Hm. He and Ink can probably make their own way back? And this world's kind of cool, but...

"I need to talk to her a bit. If we decide to stay a while, would the offer still be open later?"

"You will not be any less of your own world no matter how long you stay here. If you choose to return home, find me at the Burnt Tower."

She vanishes.

"Thanks!" he calls out, and then turns to Entei. "Thank you, too. I would've had a lot of trouble catching up to the other, and my friend's not really diplomatic..."

"Of course," says Entei, still perfectly motionless and radiating contentment.

"I'll leave you to it, I guess."

And he goes to find Ink. The consensus is to stay here for a time - they might be able to find a way to link the worlds, which'd be cool, and new places are always neat, and it's not like they're expected back anytime soon in their world. She's on summer break, and they both told everyone they were going far-traveling, so.

Onwards! To adventure!