Starkiller – the original Mary Sue (after Luke Skywalker bent a molten blaster bolt around a corner, and through a tiny hole in The Death Star, while flying an X-wing in space. And all despite having next to no training in how to use The Force.)
It did help ruin the franchise, but it was already ruined by silly novels and badly-thought-out comic book adaptations. The EU did enough damage to the universe that the trajectory was bound to hit “JEDI SUPERHERO WHO CAN WRESTLE STAR DESTROYERS OUT OF THE SKY” territory.
It’s part of why The Force Unleashed and The Last Jedi had such a negative backlash. The Force Unleashed copied and pasted the structure of A New Hope, which was enough for some people to buy, but I heard countless musings from people, whining that the characters were not as jumpy or powerful as Starkiller – with Starkiller being the one character mentioned for reference. The Last Jedi, for some people, is their favourite Star Wars film, for others, it wasn’t. What I didn’t like about TLJ’s backlash is how much people were moaning about it being different to the rest of the franchise, when it is about as derivative as The Force Awakens was.