Whenever I play Fate/Grand Order, one thing I often hear when I use that game's version of Nero Claudius is her using 余 when referring to herself. Looking it up shows me that this particular pronoun has an archaic and formal flavor to it, as well as having an oratory function. That last bit is what's kinda got me scratching my head.
In what way is 余 oratory? What the proper way to use and understand 余, as a pronoun, and in what context is it most often used?