It seems like people are having problems agreeing on the central theme of Rewrite, so I guess I’ll chime in with what I have.
Rewrite’s central theme is power. This is something that’s brought up in every route, but people often don’t catch all of it at first glance, which is why its recommended that you read it more than once. This is also a part of the why the routes seem so inconsistent; because each route focuses on a different aspect of power, which unfortunately means that other areas might have received less time and resources. Now before I start, let me state that there is a canonical route order, which should hopefully be apparent below.
Starting off with the Common Route, I’m pretty sure everybody agrees here that Kotarou has power. The problem here is that he doesn’t use it. The only times Kotarou uses his abilities in the common route is when his survival is at stake; i.e. he has to use it. He knows that there will be consequences for using it, so he is avoiding it as much as possible. This “fear” of his power is what leads him into the first of the main routes.
Kotori’s Route deals with the consequences of NOT using power. Honestly speaking, Kotarou is a failure in this route. He only used his power when needed; which left him too weak to do anything when he needed to protect more than just himself.
As if that wasn’t enough of a reason, This leads Kotarou into Shizuru’s Route, which deals with the reasons you need power. In Shizuru’s route, Kotarou learns that there are people he wants to protect; things he wants to see happen. The only means for doing these things is by attaining more power. It’s probably worth noting that Kotarou is also a failure in this route; as he was unable to protect everyone, and in fact ends up sacrificing himself for what little he did do.
Though he now knows that he will have to use his power, he is still hesitant to use it; afraid of the consequences. Fortunately for him, Lucia’s Route deals with the consequences of power and how to live with them. It seems like most people understood this part the most, so I probably won’t spend as much time explaining it; but the key here is that unlike most other routes where he learns from himself/his mistakes, he learns his lesson this time from Lucia, which makes him much less hesitant to use his powers after seeing someone else do it.
Now that he is more willing to use his own powers, he jumps right into Chihaya’s route, which deals with power’s usefulness and what can be done with it. No longer hesitant, Kotarou now uses his power almost every chance he gets. This is where Sakuya comes in though, and shows Kotarou that he should be using his power for a purpose. Though he is still not immune to making mistakes, the major turning point in this route is when he decides what to do with his power.
Now that Kotarou has overcome his fear of his powers and knows what to use them for, what comes next? control. Akane’s Route is all about controlling his powers, making them do what he wants them to do. The easiest part of this to see is with the form of his Aurora. It has become a beast with a mind of its own, only tied to him because he gives it life. Most of the time when he uses it, he struggles to keep it under his control while still allowing it to fulfill its purpose. By the end of the route, he is able to fine tune his powers to do what is necessary. (think evolution scene)
side note: Control also plays a big part in the route itself (not including the overarching plot), and Kotarou and Akane juggle it all around between each other and some of the great Gaian leaders. I would love to write another essay on just that, but I’m trying to keep this short and concise, so I’ll leave it out for now.
This trend does continue into Moon and Terra, but albeit in a slightly different way. By the time we get to Moon, Kotarou has basically mastered his powers, which leads him to ask some of the only questions left: why does he have his powers? where do they come from? and other things of the sort. to anyone who’s read the route, it should be pretty obvious that the first half of Moon deals with these questions. The second half, however, becomes a showcase of everything he has learned in all the other routes.
Side note: It’s stated in moon that the 5 routes are not the only lives he has lived, and that there are many others. The reason only these 5 routes were shown is because they are the ones that had the greatest impact on him.
Rather than look at why he should use it or what he should use it for, Terra looks at the worth of Kotarou’s power. Every encounter in Terra has both a value and a cost, both in large scale and immediate outcome. Many times in this route Kotarou is forced to sacrifice his current wants in order to move closer to completing his long term goals. It’s only towards the end where he starts rewriting himself with reckless abandon that his short term gains outweigh his long term goals.
And there you have it, a central theme broken up into how its supported by each route. I should also state that this is by no means the only central theme, and this is definitely not all the evidence there is to support this theme; so there should be a lot of room for discussion on some of the other themes/supporting evidence out there, so have at it!
Bonus: Oppai route is basically an alternate scenario in which Kotarou is given near infinite power, absolute control of said power, and little to no consequences to using that power. Needless to say, I think he had fun playing around with those abilities.