This looks great! I flipped and read through it, but didn’t actually use it in play, so take that into consideration.
Stream of consciousness thoughts:
Most of it works really, really well. I was thrown by the Shield having two checkboxes for a moment (you can’t check the second one), but that makes sense. Having clickable checkboxes is far easier than writing and erasing, too (for example, for Experience)!
Being able to choose between Backgrounds and having those populate the fields below works really well. This kind of touch makes using this really quick and really space-efficient, which is fantastic.
Under “GM Stuff”, the “Damage” box is too small, and doesn’t line up quite right with the one beside it.
The spreadsheet is nicely set up so that it’s not tempting to do anything game-breaking, but some more space for notes could be useful, perhaps (not just at the bottom, but in other spaces, too). I’d have to try it in actual play to see! Having the character sheets that vertical could be awkward (as opposed to having them as rows or as separate tabs), or it might be just great; I’m not sure.
(I’m sure people will find ways to ruin the spreadsheet, but they should be able not to!)
I have no idea what +advantage or -disadvantage does for monsters. They don’t roll, do they?
Some of the font size (especially the reference material) is really small, but I suppose someone with poor eyesight might have some Zoom settings engaged on their browser or Google Sheets settings already.
Excellent work! I would definitely use this. Lots of nice little touches, like how the Debilities, when selected, highlight the relevant stats.