Do we truly know anything about the Prime Jedi?
Honestly, it is hard to think that we know a whole lot about the first sith lord Ajunta Pall. But when we compare it to are knowledge of the Prime Jedi, Pall looks like he has his own biography.
Other than the fact that he was the first jedi and his picture appeared at Ach-To multiple times, we know nearly nothing. What are your thoughts on the power, history, and death of the Prime Jedi.
It's possible that he's made-up. It's also probable that there are too many glorifying stories about him or her or it for there to be an objective way to know who or what he/she/it was. Luke talks about the ancient texts, the beginnings of the Jedi order as something so surrounded in glorified past, that no real information can be taken from it. So the Prime Jedi could just serve the story as a reminder of that.
Another possibility is that the Prime Jedi originally wanted balance between the dark and light sides of the Force inside all of its users, which was a concept which eventually became lost over time as the Jedi got more and more dogmatically afraid of the Dark Side.