Simply complete Guest Star ???? as any character, and all currently available Dream Friends will then be unlocked for future playthroughs of Guest Star.
Dream Mode
When you have beaten Soul Melter EX you will unlock Dream mode which lets Kirby have his look from 'Kirby's Adventure to Tilt 'n Tumble'.
Extra Modes
Defeat the last boss and watch the credit roll. You do not have to complete any extra stages.
Extra Planet Delta
Clear Story Mode once.
Hidden Characters
After unlocking The Ultimate Choice mode, clear it on any difficulty. This will allow you to choose King Dedede, Meta Knight, and Bandana Waddle Dee as selectable Allies in future attempts.
Hidden HAL room
Like in past Kirby games, there is a hidden HAL room to find near the end of the game. Before going through the shiny star door in the very last room, fly up toward an "X" shaped hole in the background. You will see a tiny bright yellow star in the far-off space. Press [Up] to enter the HAL Labs room. The room contains many power-ups, and there is a large "HAL" written in blocks. Grab the Festive Dance enemy and use the super-power to smash all the blocks at once, summoning another set of HAL blocks with extra lives and other power-ups.
Secret levels
Complete the game to unlock Ability Planet (where you can grab any Copy ability) and Extra Planet Delta in the Far-Flung Starlight Heroes section in Story mode. Towards the end of the Extra Planet Delta level, you will reach a large background block with a cross and two dot platforms with an empty space above. A door will appear in the empty space. The background with the cross and dots is an original Game Boy. Go through the door to reach a hidden Game Boy style level. At the end of the Game Boy style level, you will reach a large button. Stomp on the button to summon a giant statue of Kirby, with the words "We [Heart] Kirby" over his head. If you are using the Switch Joy-Cons, the HD Rumble will play a lo-fi version of Kirby's theme from the original Kirby's Dream Land on the Game Boy.
Soul Melter EX in The Ultimate Choice
This mode is by far the hardest one of the game.To play it,you have to beat Soul Melter and Parallel World Heroes,then input Up-A-Select (-) when highlighting the option Soul Melter in The Ultimate Choice.
Theater mode
Complete the game succesfully to unlock Theater mode.