Verified answer

Blooper and Hammer Bro are the final two playable characters, though they are locked at the beginning. Unlocking them is pretty simple, but time-consuming: the first time you clear the Star Battle Arena, you'll wind up unlocking one or the other at random. After this, you'll need to clear the Star Battle Arena again using whatever character you unlocked. So, if you unlocked Blooper the first time through, play as him the second time through, and you'll get Hammer Bro.

nope, beat star duel twice to get both secret characters, blooper and hammer bro