Pokémon Pokopia: Get Phione
Phione is one of the Mythical Pokémon introduced in the Bubbly Basin expansion for Pokémon Pokopia. Unlike most residents, Phione does not appear simply because you build its preferred habitat and wait. Phione hides inside an underwater cave on a Basin Dream Island, and the game randomly decides whether it will appear during each visit.
Quick answer: Visit the Starmie Doll Dream Island in the Basin area, then dive into the cave beneath the island's central mountain. Break the stone wall and check for pillars behind it. When Phione appears, speak to it and choose "Let's go see everyone!". Return to Pokopia and place Phione in any Basin habitat.
Find Phione
Phione is connected to the Starmie Doll Dream Island, which takes you to a Basin-themed location. You could also encounter Phione when using the Ditto Doll or Substitute Doll because both items send you to a randomly selected island. However, the Starmie Doll gives you the most reliable chance of reaching the correct island type.
Phione's appearance is affected by two limits. You can visit only one Dream Island each day, and the island layout changes randomly whenever the island is generated. The presence of Phione is also random, so several attempts may be necessary. Try advancing the day through time travel if you want to repeat the search sooner.
Confirm Phione
Reach the underwater cave. The central mountain is your main landmark after arriving on the island. Swim toward the mountain, dive below the water, and search around its base for a cave entrance. The opening is directly beneath the mountain and becomes easier to identify once you are underwater.
Check behind the stone wall. Inside the cave, find the wall made from stone blocks. One section contains a small opening or gap. Swim close to the gap and look through it. If pillars are visible on the opposite side, Phione is on the island. Break through the wall to enter the hidden area.
If the gap shows no pillars, Phione is not available on that island. Wait until the next day, or use time travel, and visit another Starmie Doll Dream Island. Must repeat the cave check until the pillars appear.
Return With Phione
Collect the Yellow Poké Ball. Before you speak with Phione, pick up the nearby Yellow Poké Ball. This item contains three Rare Pokémetal, so it is worth collecting before you leave the cave.
Invite Phione along. Walk up to Phione and start the conversation. Select "Let's go see everyone!" when the choice appears. Phione will then follow you and remain with you as you return to the main world.
Return with Drifloon. When the game asks whether you want to return to Drifloon, choose "Yes". This keeps Phione in your group during the journey home. After returning, speak with Drifloon again and travel to the location where you placed the Starmie Doll.
Build A Habitat
Phione continues following you after you leave the Dream Island, so you must prepare a suitable place for it to live. A basic Basin habitat is enough at first. Basin Tall Grass works as a simple starting option, and you can improve the habitat later.
Stand beside the habitat, speak with Phione, and choose "Wanna move in here?". Phione will settle into the area after you confirm this choice. This is the final step required to move Phione into its new home.
You can keep Phione in Bubbly Basin if you want an underwater-themed area. Could also place Phione in another region because Mythical Pokémon provide a stronger Comfort Level bonus than ordinary Pokémon. That bonus improves the area's Environment Level and makes reaching Level 10 in each region easier.
Phione is now settled on your island as another Mythical Pokémon. When a Dream Island does not contain Phione, the process remains the same: advance to another day, load a new Starmie Doll island, and inspect the underwater cave until the pillars confirm that Phione is waiting inside.