I can't really think of a use for one-off characters right now, but Lazy Devs floated the idea of making a little editor for them in his recent video and that seemed like a fun short project, and I'd never done anything with GUI elements before.

Meant to be used offline.
Not all features will work off the BBS but you can use the basic functionality.
Multiple persistent slots for editing.
Really it's just a page of the spritesheet you're editing.
Per character UndoRedo
This is limited to 32 levels of undo per character but there's no particular reason it should be. Basically like the Pico-8 sprite editor (plus Rotation) though I didn't feel any need to implement the more complex drawing things for the 8x8 monochrome format.
Flip, Rotate, and Shift
With some keyboard shortcuts you can see in tooltips. Shifting uses the dir buttons.
CopyPaste One Off Character Strings
That's the whole point of it. There's a toggle to use hex or binary, but just use hex because 'binary string' doesn't really make any sense; if you have an empty row you're going to have 0's in the string and no string workflow is going to enjoy that; afaict you really can't even paste them into a cart which seems like the #1 thing you'd want to do; just use hex.