Use this list of unlockables:
Kendarie Warrior for versus Mode: defeat the Kendarie Boss on Kendarie Fortress Area 1
Helina for Versus Mode: defeat the boss in the Scared Arena and find Helina in an arena to defeat her
Get character's Neutral Skill is 6 or more: defeat all bosses in the first arena
Rashiannu for Versus Mode: complete the Royal Tower Upper Floor in single-player mode and save the game
Thalnos for Versus Mode: complete the game once and beat Leod and Helena in the Sacred Area 2, then beat Thalnos in the Northern doorway of the arena then save
Urbur Cultist for Versus Mode: defeat an Urbur Cultist and finish the current stage, then save the game