I've got this bit next to my back door that looks like it was never properly finished. There doesn't seem to be any structural issue, so I'd feel comfortable fixing it myself.
Trouble is, I have no idea what to do there. Plaster a corner? Put some drywall in? It's a strange shape and I am not sure where to start.
EDIT: As requested, a picture of the top corner and the left side. (This is really a lovely house, overall, I swear! Despite the insanity of a few things like this)