First complete Arcade mode. After you defeat Arcade mode with a different character each time, you will get yet another new and different character. Once you beat Arcade mode and unlock all characters, There is one left. Venus should be the last one you unlock from Arcade mode. After you acquire Venus, Beat Marathon mode to unlock Aphrodite.
Successfully complete arcade mode with all nineteen characters.
Top Collector mode
Successfully complete arcade mode with ten characters. In this mode, you have to collect as many bras as possible as Venus