My favorite weapons were the laser and parasites.
The parasites are probably the best weapon against the bunny boss he has no change when you have it level it up. Laser is the best weapon in the game, not only due to damage but It ALWAYS hits the target no matter how fast they are including the FAST bosses.
I did find the secret which mentions the character name, Izu Kamiya.
https://i.imgur.com/hBCpmv8.png
Also found the M%005 Space Helmet, what is that helmet for?
https://i.imgur.com/OxRLwc8.png
What I did not like was how hard is to get HP which makes some upgrades basically useless.
Didn't see a point in the Shift key that would have been better if it was a temporary boost in speed.
Also did not like the max level... Would have liked to be able to get ALL skills.