So, I'm in esmerald city (or whatever it's called), the city of the sixth gym.
I've visited the gym, but the owner isn't there, so I went to the power plant..
But he isn't there either.

What do I do now?

Close the game and play Minecraft?

Haha, kidding. Put on the Team Steam uniform and go inside the botanical building. Them defeat TS and you'll find Esmerelda I think.

That's at the third gym or something. I'm at the sixth gym and the city with the power plant. :p

Download the walkthrough.

Oh. Right. You said Esmerald Gym, so I thought you meant this one. :P

Fight your way through the short Power Plant and talk to Rayner, standing with his Electivire at the far right side. That should be it.

The problem is that he isn't there. =/ (Rayner, I mean.)

Go towards the east where you come up with a block at the end of a long path. You'll pass a Daycare centre as well. Then go back and check the Power plant.

A block at the end of a long path past the daycare? Alright, I'll check it out. :p

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Wait, what block where now?

Try to re-load the ROM. As it's a bug where in Rayner is not showing, but he's really there.

Saved, closed and restarted. Found him!