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.

Platinum fighters:

Select "SD Hiryu" mode. Beat any platinum character to earn a medal. The platinum characters will be playable you win all ten medals.