Muse Aquarius is the fourth Muse to be introduced in the Lucidity arc of the game series, and the alter ego of Amy Siden. Her representative song on Zenith's debut album is Confused in the Ocean, so it is only fitting that her powers (and her Muse name) revolve around water.
- Muse Aquarius Staff (her main weapon)
- Her bare hands
- Fishtail Slap (basically, a simple karate-like routine that takes away only 1 hit point per second of performance; however, the amount of enemy HP that can be lost is indefinite)
- Tsunami (the Muse Aquarius Staff, after achieving a hit combo of 45 or more, releases a sonic boom-like burst of blue energy that takes 75 hit points per enemy, with no limit on how many enemies are affected)
- Sea Mist (the staff generates a thick, eye-stinging fog that blinds enemies for approximately 10 seconds)
- Glacier (creates a thick protective barrier around you and up to two other Muses; does not affect enemies, but lasts longer and if more affective than the other Muses' shields)
- "Muse Aquarius Evolution!" (when she transforms)
- "In the names of both Neptune and Zenith...be drowned!"