Beleriphon
Totally Awesome Pirate Brain
Some RPG groups don't have rulesets, leaving it all to the GM instead of rulesets.
One of the oddest I've played, you play a corporation, not a person.
And I've got a couple games that I've only played solo, and several more where most of my play was solo. And I'm not counting the ones that ONLY have mechanics for solitaire play. Runeslayers I've only played solo, for example — there's a solitaire module in the core — but I've never gotten it to table with players. Most of my play of TFT has been solitaire; in hours, probably 10:1; if we break out group RPG play from tactical battles, S:G:B::302. Same for T&T, tho the ratio is much closer to S:G:
1.
Each of your minimums is able to be individually falsified.
In several games I have played, characters are created as part of the process of play, so those are not necessary for play to start.
As for the "agreed upon set of rules..." my first game of D&D, ever, I played blind. I didn't have a character sheet. I had not read the rulebook. I agreed on the rules in the sense that I trusted my older brother to implement all rules in the background where I didn't see them.
So.. you need a group of people - and the size of the group may be 1.
I would suggest that a new player may not know the rules, but they implicitly agreed to abide by them, even if the rules are the GM makes them up as they go along. That's still a ruleset, just one that is opaque to one side of the play group.
One player can still be a group, and you still need a character of some kind to play an RPG. Sheets and other ephemera aren't inherently necessary despite how useful they are to facilitating play.
Thus, I still assert a group, a characters, and rules needed to play an RPG. The specifics of each of those can and do change substantially depending on what kind of game is being played and the general expectations that changing each one produces.