The character is none other than the mysterious Fleet Admiral named Gallius Rax, and in Life Debt we get his backstory. A lot of things match up to him being Supreme Leader Snoke during the time of The Force Awakens…
Rax is an orphan living on Jakku during the time of the Galactic Republic and he is befriended by Sheev Palpatine who appears to be doing Sithy things on the planet.
Gallius has Force Sensitivity and is actually able to resist the power of Palpatine, which of course makes him very valuable to the future Emperor.
Rax becomes a puppet master and special advisor to the Emperor. He’s the one pulling the strings behind the scenes.
When Rae Sloane looks at him, it’s like she’s looking at a hologram.
He’s described as being very pale and his eyes as black holes.
He believes it’s very important for the Empire to recruit children and indoctrinate them at a young age. This is exactly what the First Order does as opposed to a clone army in The Force Awakens.
By the end of the book he’s calling himself Emperor Rax.
It should be noted that the book doesn't come out and name Gallius Rax as Supreme Leader Snoke, but this may show up in the third book in the trilogy coming called "Empire's End." It should be noted that Life Debt ends right as the Battle of Jakku is about to kick off.