I am not 100% sure if this works, but it would be interesting to try out. (edited for Spoilers) 

1. Start by changing to Stage Select rule. You’ll want the fourth option, “Junguri.”

2. Now return to the Brawl menu and select Special Brawl. It’s the one that shows an image of a Mario hat with bunny ears.

3. Next change the first category (the one with the % symbol) to Tairyokusei and and the third category (the one showing the bunny ears) to Hana (it’s the first option). Hit “okay” at the bottom of the screen. The game should take you immediately to the character select screen.

4. Now select your character and a character for the computer. When you select the computer’s character, you’ll notice the number 150 appear next to the image at the bottom. Move the pointer over the 150 and hit “A.” Now push up on the control stick to reduce the computer’s HP to just 1.

5. Start the match. The battle should last all of one second.

6. Repeat step 5 until a new challenger appears. Defeat the new challenger  to unlock him. After about two or three hours of this, you’ll have all the characters unlocked including the final hidden character …….. and his obscure sidekick ………..

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