Secret Ending Scene
Defeat the True Final Boss as both Gunvolt and Copen. Then, you need to complete at least 80% of the Challenges as both characters (80% each). After completing enough Challenges, the next time you defeat the True Final Boss (as either character) a special scene will play after the credits and Save Screen.
Shovel Knight Battle
In the amiibo option in menus, you can use the Shovel Knight amiibo. This sends you into a screen where you pick Gunvolt or Copen and then face Shovel Knight with the chosen character at level 1. Winning as Gunvolt unlocks the "Shovel Ring" in the synth menu. You only need 10,000 Yig to make it and equipping it lets you perform one air hop and one air dash at no EP cost. Beating Shovel Knight with Copen adds the "Code of Shovelry" to your synth list. It also costs 10,000 Yig to make. It has a size of 5. When reloading in the air, you'll bounce off the ground high into the air and recover all spent Bullits as usual. It's extremely helpful for staying in the air and you'll be able to reload with no downtime.
