I've chosen Kylo Ren as my successful character for quite a few reasons: Firstly, the new movie is still ingrained into my brain because it was just really good. Lived up to the hype in my opinion. Kylo Ren is a very interesting character when you look at the marketing they did for the film and the film itself. I mention the marketing only because they were clever and tricked us, like Kylo Ren tricks others in the movies.
We thought Kylo was going to be a menacing Darth Vader clone with a cooler lightsaber, what we got was an angsty teenager. You'd never know Kylo's true personality from the trailers or toys because they make you think exactly what Kylo wants others to think of him in the movie - that he's cool and evil and menacing like Darth Vader.
There are some hints to his personality - his lightsaber, for one. It's unstable and poorly made, sort of like Kylo, who's temper gets the best of him. The lightsaber is so poorly made because Kylo never had anyone to properly teach him how to build it.
While Kylo wants to be Darth Vader so bad he's actually almost the opposite. Kylo is young while Vader is old. Kylo's temper gets the best of him but Vader is always calm, cool, and collected. Vader's face is horribly scared and Kylo's is perfectly fine. Kylo has a loving family, Vader had no one.
I think Kylo is a very successful character because you can learn a lot about him thru actions rather than words and the visual implications of his lightsaber are clever.