In the second hs book, pg 281, hussie says having multiple copies of a character operating in totally different circumstances is for characterization but also "proves to be inseparable from one of the story's essential themes." What did he mean by that? What theme?

He’s referring to the theme of Identity! Of who we are, of how we grow as people and growing up. Terezi for example is a PRIME example of this, in which she regrets her choice and wonders if it even mattered given how Paradox Space operates, for the ENTIRETY of the Pre-Retcon part, and once she has a chance to fix it and thinks she’s going to make it better, this new Terezi STRUGGLES with not being able to live up to Game Over! Terezi.

It all boils down to talk of the Ultimate Self again, how all these choices and events do have repercussion on other copies of themselves, how the core of their character is an amalgam of every iteration of themselves and what they do.

… Which actually just made me realize something, I may elaborate on this in a little bit.