From the DEAD OR ALIVE++ article on the Free Step Dodge wiki: https://www.freestepdodge.com/wiki/dead-or-alive-plus-plus/

Unlock Raidou Complete Tournament mode as Kasumi. Then, on the Name Entry screen, enter the name SIAWASE? and Raidou will now appear selectable on the Character Select screen.​

BURST Mode BURST mode will make the entire floor a danger zone. To play with BURST mode enabled, press and hold down all three buttons (:H+P+K:) before pressing the START button on the title screen.​

ENDLESS Mode Endless mode is the closest thing to a training mode supported in the game. It can be enabled under Game Options in the game's Test menu via the test button. After selecting a character, the player will be against a randomly assigned opponent unless the second player has selected a character. The timer will never tick down, and life is replenished once a combo ends. You are also unable to enter BURST mode while Endless mode is enabled. Pressing the START button will frame pause the game.​