- Polar Star can be traded up for ONLY one of:
    - Machine Gun                   upgrade at Labyrinth Shop: Turbocharge
    - Snake                         upgrade at Labyrinth Shop: (snake)
    - Spur (aka Charge Shot)        upgrade at Labyrinth Shop: Whimsical Star

You can only get one of these:
    - Booster v0.8
    - Booster v2.0

You get the "bad ending" if you answer "yes" to Kazuma's question in the Egg #00 chamber

- You can't rescue Curly if you get the Booster v0.8