As a relationship, Nia/Simon is pretty piss poor, let me just get that out of the way. I don't like the couple, didn't find it moving on a personal level, and outright guffawed when it turned out Simon's "deepest inner desire" didn't even have Nia anywhere near it. (I don't ship Simon with anyone else, that just struck me as amusing considering the circumstances) Their actual romantic interaction is limited to a few scenes, and we never get a concrete sense of what the one means to the other, aside from the fact that they are in Love.

However, the entirety of Gurren Lagann tends to use emotional shorthand like this, as you know. The characters are more important as embodiment of themes and ideals than as actual characters, with notable exceptions being Rossiu and Kamina.

In that respect, there is one interesting, unique thing about Simon/Nia, and that is his refusal to override her choice to die at the end of the series. Nia, as someone who had been used her entire life, including by the narrative in regards to Simon, got the ability as last to choose her own destiny, and Simon loved her enough to let her go.

That's pretty much it.