Play as Bokuchin:
Beat Bokuchin during the game to enable him as a playable character.
Play as Ryumaou:
First activate the Ryumaou Tournament by earning nine medals. Then win the tournament to enable Ryumaou as a playable character.
Very Easy difficulty:
Highlight the "Easy" difficulty option, and keep pressing Left until "Very Easy" comes up.
Very Hard difficulty:
Highlight the "Hard" difficulty option, and keep pressing Right until "Very Hard" comes up.
Select "SD Hiryu" mode. Beat any platinum character to earn a medal. The platinum characters will be playable you win all ten medals.
Thanks to Revolution readers Kevin J., Dustin Beyette, and Randy Malone!