There are six hidden characters that can be unlocked to be used in challenge mode and against another player. What you need to do is highlight a certain character and press the C button three times within a second.
It isn't as bad as you think though, since you can choose which second in the countdown to press the C button three times.
20 - Judgment
19 - Sun
17 - Star
15 - Devil
13 - Death
11 - Justice
8 - Strength
7 - Chariot
6 - Lovers
5 - Hierophant
4 - Emperor
3 - Empress
2 - High Priestess
1 - Magician
0 - Fool
If done correctly you will see the hidden characters appear about a second later. The characters unlocked this way are: Moon, Hanged Man, Hermit, Temperance, Tower, and Fortune. You will also gain 10 seconds to the countdown, which can go up to the max of 20.
Hit B on any character to get their alternate colors.
If done correctly, the hidden characters will appear. The characters unlocked this way are Moon, Hanged Man, Hermit, Temperance, Tower, and Fortune. You will also gain ten seconds to the countdown timer, which can increase to a maximum of twenty seconds.