Fulfill the following requirements while playing story mode to unlock the corresponding character. Note that these all must be done with the difficulty set to normal or hard.

Unlockable	How to Unlock
Civilian Blimp	Harm a civilian blimp when they appear on either stage 10 or 14.
Civilian Child	Harm a civilian child when they appear on either stage 10 or 14.
Civilian Elder	Harm a civilian elder when they appear on either stage 10 or 14.
Civilian Wimp	Harm a civilian wimp when they appear on either stage 10 or 14.
Civilian Woman	Harm a civilian woman when they appear on either stage 10 or 14.
Cyclops	Defeat Cyclops (multiple stages).
Earth Spirit	Defeat Earth Spirit on stage 24.
Gambo	Defeat Gambo on stage 9.
Gargoyle	Defeat Gargoyle on either stage 19, 23, or 24.
Gash	Defeat Gash on stage 11.
Giant	Defeat Giant (stage 19).
Glute	Defeat Glute on stage 9.
Goblin	Defeat Goblin (multiple stages - primarily stage 3).
Golden Silver	Defeat Golden Silver on stage 30.
Golem God	Defeat Golem God (multiple stages - primarily on stage 5).
Han the Hero	Defeat Undead Rogue on stage 8 while playing as Han.
Kanon	Defeat Kanon (multiple stages).
Katrina	Defeat Katrina on stage 3.
Loper	Defeat Loper on either stage 23 or 24.
Macho	Defeat Macho on stage 9.
Mid	Defeat Mid (stage 3).
Proto Silver	Defeat Proto Silver on stage 4.
Rebel	Defeat Rebel on stage 5.
Royal Blade	Defeat Royal Knight (multiple stages).
Royal Knight	Defeat Royal Knight (multiple stages).
Royal Mage	Defeat Royal Mage on stage 2.
Royal Mech	Defeat Royal Mech (multiple stages).
Royal Purger	Defeat Royal Purger (multiple stages).
Serena	Complete the game with any character and get any ending.
Skeleton	Defeat Skeleton on stage 2.
Sky Spirit	Defeat Sky Spirit on stage 29.
Sky Warrior	Defeat Sky Warrior on stage 29.
Super Zur	Defeat Super Zur on stage 25.
Undead Rogue	Defeat Undead Rogue on stage 8.
Valgar	Defeat Valgar on stage 28.
Wolf	Defeat Wolf on stage 3.
Xenovia	Defeat Xenovia on stage 24.
Zombie	Defeat Zombie on stage 25.
Zur	Defeat Zur on either stage 5 or 25.