Snoke's other apprentice
An acolyte served as the student of Supreme Leader Snoke, a shadowy and puzzling person strong in the dark side of the Force. Unbeknownst to most, Snoke was a genetically engineered being, artificially created by the Sith Lord Darth Sidious and his adherents. In the time of the New Republic Era, this student learned from Snoke before he developed an interest in the Skywalker bloodline and ultimately chose Kylo Ren to be his next student.
Behind the scenes
This individual's existence was hinted at, though not directly, in Star Wars: The Force Awakens: The Visual Dictionary, which stated Kylo Ren was Snoke's most talented student, suggesting Snoke had instructed others. Star Wars: The Last Jedi The Official Movie Companion subsequently verified that Snoke had indeed trained at least one other person.