Star Wars has subtly confirmed the first Jedi did not serve the light side of the Force at all; rather, they were agents of balance. The sequel trilogy shone a fascinating light upon the ancient history of the Jedi Order. In Star Wars: The Last Jedi, Luke Skywalker had hidden himself away on the remote ocean-world of Ahch-To, site of the first Jedi Temple. There, viewers caught a brief glimpse of a mural representing the Prime Jedi - the first Jedi Master, founder of the order.
The mural revealed the Prime Jedi was a member of an unknown race, possibly related to a species called the Zeffo, and he was shown wielding a proto-lightsaber. Most interesting, however, was the fact the mural presented a very different philosophy of the Force to the one seen in the Skywalker saga. Light and dark were given equal prominence in his depiction, and he was seen as representing the darkness in the light and the light in the darkness. In other words, the Prime Jedi appeared to be an agent of balance.
The recently-published Star Wars tie-in Secrets of the Jedi appears to support that view. This is an in-universe fact book, supposedly written by Jedi Master Skywalker himself, and it contains notes running all the way up to his death. Naturally, there is an early section dedicated to the discussion of the Force, with Luke discussing the idea of Force alignment. "Those of us inherently inclined to harness the Force's vast energies must make a difficult choice," Luke notes. "To use its powers either for the greater good or for personal gain." This forms the basis for the light and dark sides of the Force. Almost as an afterthought, Luke moves on to discussing the idea of balance, suggesting there is a valuable lesson to be found in this. Significantly, he attaches an image of the Prime Jedi mural as part of the section on balance. The clear implication is that he associates the Prime Jedi not with the light or dark sides of the Force, but with balance.
This fits perfectly with Cavan Scott's audiobook Dooku: Jedi Lost, which hinted the ancient Jedi saw balance as a distinct aspect of the Force, like light or dark. Dooku had learned an old mantra, one taught by the first Jedi and designed to be used as part of meditation:
"We call upon the three — light, dark, and balance true. One is no greater than the others. Together, they unite, restore, center, and renew. We walk into the light, acknowledge the dark, and find balance within ourselves. The Force is strong."
Secrets of the Jedi suggests Luke Skywalker didn't accept this teaching himself, otherwise he'd have made a lot more of the balance of the Force. But Rey has been learning from the ancient Jedi texts, ones that date back to the very founding of the Jedi Order, and thus she is likely to have absorbed these early ideas. There have been hints some of the new Force powers seen in Star Wars: The Rise of Skywalker - the likes of the Force Dyad and Rey's new Force healing - are associated with balance, explaining why they've never been seen in the canon before. If so, hopefully Star Wars will eventually reveal what Rey's new Jedi Order looks like, and it will be a lot more similar to the Prime Jedi's teaching.