The word DARTH comes from 2 words... DAR-k and si-TH. See what I mean ? Also, yes Dooku/Tyranus was the dark apprentice of Palpatine/Sidious. Ventress was called a Sith Assassin and Sith Agent but she was never a true Sith Lord/Lady. Anyone can use a red Light Saber if they figure out how to make one or are given the weapon. This dark warrior may have the regular name... Kylo Ren but it's rumored that he is also called DARTH RUIN. They don't want to give to much away so soon. A fun fact is, years ago there was a Star Wars cartoon series called DROIDS and in that show there was a guy called... Kybo Ren. The name could just be something as simple as changing a letter or 2 to make a cool sounding new name like in the CLONE WARS cartoon there was a Mandalorian Duchess named Satine and in the new STAR WARS REBELS cartoons there is a Mandalorian Warrior girl named Sabine. The 2 could be related but that wont be talked about until Season 2 maybe.
Other designs the crew worked on included a fortress for the Jedi Killer. Concept artist took "iconic shapes that we know from " but put "different material on them—in this case turning them into concrete instead of that slick painted metal". The crew also considered giving the Jedi Killer his own droid, such as a torture droid or something else that looked ominous. The idea of the Jedi Killer having an interrogation droid ultimately made it into the film, with Kylo Ren using interrogation droids and other torture techniques to break his prisoners.
If The Emperor is still alive, and the Rule of Two is reinstated, this could be the Apprentice you’re looking for. But the best – sketchy – reason we can give this theory some air time is, well, he wears a hoodie like Kylo Ren’s.