There is another…
While Star Wars: The Last Jedi shorted fans on backstory and information regarding Supreme Commander Snoke, once again further information is revealed from tie-in merchandise.
A book that was released with Star Wars: The Last Jedi has a chapter on Snoke where it is stated: “Force sensitive and highly attuned to the Dark Side, but not a Sith, Snoke has trained Kylo Ren and at least one other apprentice.”
The book doesn’t go into further detail, and it is anybody’s guess who is Snoke’s other apprentice, or even if the apprentice is still alive and kicking in the galaxy.
It also remains to be seen if the movies will ever mention the second (or first?) apprentice of Snoke as Star Wars: The Last Jedi seem to be stating they are moving past the character.
The book further mentions Kylo Ren being master of the Knights of Ren, which was foreshadowed in the Force Vision seen in Star Wars: The Force Awakens, but not featured in Star Wars: The Last Jedi as well.