Defeat Mildred before time runs out in the last battle if the timer is running. Note: Do not win by having more life than her (with the timer on).
Bonus Gallery Picture
Play a character 10 times in any mode to unlock it.
Defeat Mildred with all characters and get the good ending.
Story Mode Artwork
For playing story mode for each character in the game you will unlock the character artwork for the story events for the character you picked. And you will get the character ending art for also beating story mode for the character you chose.