First you take Cave Story, then throw in a frog. Add a bunch of black monsters, and presto! It's Kero Blaster!
Plenty of unused graphics and leftovers from Pink Hour.
A black variant of the main character with red eyes appears a few different places in the graphics. These are usually located to the right of the blue Overtime version of the character, similar to how the yellow Omake sprites are positioned next to the green ones. This may suggest there was going to be an Omake version of Overtime mode at some point.
In item.png. He seems to be missing his red eyes here.
There is an unused Level 5 version of the Peashooter weapon labelled "Death Beam" in the words.txt of the original 1063 version of the game, and is in fact usable via save editing! It fires large pink projectiles very quickly that do a whopping 99 damage each, as seen in this video.
The Steam release of the game removes the Death Beam string from words.txt and crosses out its icon, but adds the string "Kamui" at the end of the names for Fan upgrades. The above icon also appears below that of the Level 4 Fan, Ninja. However, no other references to this weapon are known to exist and it is not possible to obtain via save editing.
This weapon does massive damage if it nails its target.
Graphics for a missile launcher and its missiles appear in the 1063 version of the game, but are mostly removed in the Steam release. The file explain.pxeve contains a description of it in all languages.
This little guy from multiple pre-release material still has graphics in the final game despite not being used anywhere. He even seems to have been touched up at some point, as the right image has slightly different teeth.
An odd graphic of the main character standing on top of the bank vault miniboss. Judging by the cursors layered over it, this image may have been used in a level editor.
A graphic for an unknown or locked achievement. The final game uses silhouettes instead of question marks.
Small graphics possibly related to a debug mode or map editor.
Leftovers from the free demo of Kero Blaster, Pink Hour. As the office lady never uses the teleporter in this game, these graphics go unused.
These graphics appear below the used ones and are scribbled out. They appear to use slightly different gun colors.
It appears that 4th variations of the Bubble, Fire, and Kuro Blaster weapons were planned at some point. The grey flower graphic does not appear to be used, either.
The 4th variation icon for Bubble actually appears very briefly in the trailer for Kero Blaster (47 seconds in).
Unused item graphics for a dot, an umbrella, an early jetpack, and a key.
An empty Life Jar. The item simply disappears upon being used, so it is never seen as empty.
Next to the icons for inventory items on the save file screen is this dark bubble, presumably for the Kuro Blaster. Since the Kuro Blaster does not appear on the user's save file, this is unused.
When the game is paused it will display a graphic related to which platform it is being played on. Presumably these stand for "iOS," "Windows," "Mac," and "Steam" respectively. However, Kero Blaster does not have a Mac OSX port.
The ice blocks are still in the freezer.
The name of this text is similar to the expTooEarly3 that is used in OXOX Hotel to give a nod to potential sequence-breakers, however it isn't referenced in any field files. It was probably meant to be used when trying to access the second Kuro Blaster before getting the flamethrower.
It's not easy being Green.
An unused normal mode equivalent to the zaxOtsukare that is used in Zangyou mode. Again, no field files reference it. Strangely, this text is present in every language except Japanese, suggesting the localization team added it rather than the developer.
Though not technically unused, there exists an event spawner named TestEpisode placed to the left of where the player spawns in the first level. This event, named expEpisodeTest prints a string that references which campaign's global event file is loaded, either normal or hard (Zangyou) mode. This string disappears once the player walks right to the next area.
This event's location is also where the player is spawned when leaving the debug room, with unused events Debug1 and toDebugRoom implying that it was once possible to enter it through the first area. The Zangyou version of this map does not include events related to the debug room.
Similarly to the debug room, the first area of normal mode is littered with sporadically placed off-color tiles. The furthest left shows where this event exists, again similar to how the debug room functions, creating an interesting continuity with a removed feature.