About Kylo Ren:
I think it is his weakness that will ultimately prove to be his strength. It enabled him to kill his own father, when even Darth Vader in the end couldn't kill his own son. Kylo Ren's fear of being weak will make him do ANYTHING.
As for complaints about Rey being so suddenly strong, there's a scene which shows exactly why: she gave herself up to the force, and it guided her to victory.
I think it is his weakness that will ultimately prove to be his strength. It enabled him to kill his own father, when even Darth Vader in the end couldn't kill his own son. Kylo Ren's fear of being weak will make him do ANYTHING.
As for complaints about Rey being so suddenly strong, there's a scene which shows exactly why: she gave herself up to the force, and it guided her to victory.