How to Find the Ruler on Pokemon Platinum

Beat "Pokemon Platinum" by defeating the Elite Four.

Fly to Snowpoint City and take the ferry to the Battle Zone.

Complete the mandatory double battle with Flint and Volkner.

Travel northwest toward the gate to Route 225 and talk to the fisherman standing there to receive the Super Rod.

Travel southeast until you reach the gate to Route 230. Take Route 230 eastward until you reach Route 229, then head southeast to reach the Resort Area.

Talk to the swimmer in the small pool, who tells you about the ruler of the resort area without going into any detail.

Travel down and slightly to the left to the bridge. While facing the water, use your Super Rod.

Keep fishing until you find a level 100 Magikarp, which you will catch eventually. You may encounter lower-level Magikarp at first, but none of those are the ruler.

Use a Quick Ball on the first turn of battle to catch the level 100 Magikarp after catching it. If it escapes, use a move such as Hypnosis to put it to sleep and then use an Ultra Ball or Dusk Ball instead.

Speak to the swimmer again to get a new line of dialogue about the ruler of the Resort Area.