He's the Jedi Thrawn aligns with in the original Thrawn Trilogy. I think he's the big threat that Thrawn has come across in this new Galaxy. My thinking is that perhaps he's from the High Republic Era and is why it fell. I think he caused a split in the Order and took his followers with him into deep space eventually establishing his own Temple. I also think he's perfected cloning and each clone is more unhinged than the last incarnation.

I think he's been leading a campaign to wipe out any force sensitive group not aligned with his views and is what the Night Sisters are running from. This explains how they know Jedi. I suspect he'll be a cult leader who has on the surface created a perfect Jedi society akin to Luke's in Legends but will have it focused on him as a grandmaster above the council.

I will also be adding some more to my theory. I think he's why Yoda was so strict on his views. I think maybe Yoda was Joruss's master but being lenient and giving into emotion led to Joruss's fall. I believe this is why Yoda was so firm in sticking to the code. I believe it was because he feared not keeping to it would create another Joruss.