I use the dude with the 2 daggers and the giant hat. That guy is awesome. A word of advice, when playing against the computer, use the weaker slash as it is faster and can make for some easy, but cheap, combos.
As for the secrets, I haven't found any.
That Hanafuda game is fun!! When I unlocked it, I was playing that more than the actual game. It's pretty easy to play too.
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"Everyone's Gonna Die!!"