Knights of Ren must first be squires to older Knights. A bit like Jedi Masters and Padawans, but more bloody and sexual.

--the knight Kylo is apprenticed to can be an original character of any gender, or any character if you want to mix up canon a bit, or a clone of an original trilogy or prequel character (Dark clone of Obi Wan, Qui-Gon, someone on the Jedi counsel, or even a clone of Anakin or Luke)