Some people speculate that Snope is Darth Plagius which was the person that Papaltine thought he killed. That seems fairly plausible given that he supposedly knew the secret to life itself (or some such). There are other that say that Snope is actually Darth Vader reincarnated or something like that. There was a picture circulating where he scars match the scars of Darth Vader. That theory doesn't make much sense.
There is also the possibility that he is some random other force user. Maybe he was standing back and watching Palpatine's rise and fall from power. We'll see, I guess.
Now this leads to the question of how she ended up on Jakku. My guess is that when Kylo rebelled and killed the new Jedi, Luke was upset at his failure and hid. Rey's mother then put her somewhere safe and Luke thought it would be better to wallow rather than hunt her. But again, we'll see.