For me what makes Simon/Nia great is that they're very clearly soulmates, we see lots of stuff about how their LOVE SAVES THE UNIVERSE, how they can reach each other across worlds--but rather than having this just lobbed at us, or supported only weakly through brief moments and occasional symbolism, we see all the little ways they interact and make each other better that lead up to it. If TTGL itself is a reconstruction of the Super Robot genre, then Simon/Nia is a faithful reconstruction of the concept of true love.

I'm working on a ship_manifesto essay that I intend to post in the next few days, but it's so long that I half suspect people who aren't diehard fans will wind up skipping over it anyway (which ruins the whole point but sooob I can't make it shorter), so...tiny tiny summary here.