COPYPASTA TIEM LOL
I have a theory that if Nia did not exist, Simon might have turned Garlock rather than Starlock. If that's a good thing or a bad thing, it's up to you guys.
I'm definitely looking too much into a character that appeared for 30 seconds, so please feel free to call bullshit on this, but I'm also basing these thoughts on what we saw of Simon's life story.
Simon's arrogance in episode 9 by trying to become like Kamina and Garlock's though guy attitude match up quite a lot in my mind. I think that if Nia wasn't there to support and help Simon face the truth that he couldn't become Kamina and had to move on, he'd slowly force himself to finally accept that; however, instead of making Simon proud of who he is and taking Kamina's will in his heart like he did, Kamina's death would probably have left a hole in his life forever, making him bitter and guilty about it.
Forcing himself to move on like that, deeply hurt and without people's proper support (because you can see the other Gurren-dan members tried to, but still thought that helping Simon was hopeless and that he had to find the anwers on his own), I can only see Simon becoming independent through arrogance - trying to compensate too much for the worthless person he thought he was, compared to Kamina, until it becomes part of him.