((Disclaimer: Only posting this here because it was asked here… Feels very off-topic; sorry about this.))
Charlotte had a pretty large focus on Yuu’s journey and development. He always had a strong degree of doubt in himself. He only ever used his power, not his ability, so to speak. (We also see this echoed, one way or another, in the other power users that the student council would target.) (Ending: His last plunder is “courage,” something that he finally gained through his own abilities. Something he didn’t actually need his power for.)
(Meta: My explanation relies pretty heavily on the story as a whole; I’d say that alone is decent evidence of my claim.)
As for what it means to me personally, I think that everyone can agree that figuring out your own identity is something that can be rather distressing. Some people will emulate others. Some people will only grab bits and pieces of other identities to make their own. (Some will try to build themselves from the ground up, but they’re already biased by their interactions with others.)
A lot of people lament their own lack of ability in various fields. Some people believe they have a lower capacity to do something than others and will often give up trying to improve their capabilities. It’s not unheard of for people to look toward others who appear to be more skilled or talented, or just generally better off than they are, and wonder what’s different, or “why am I not that person?”
That quote underlines this quite well, in my opinion.
Okay, so I didn’t really answer what it means to NotKyon specifically. Well, NotKyon often wonders why he’s not Kyon. Not everybody in a story is the protagonist, after all, as much as each character might like to be. Even so, why is NotKyon himself, and not Kyon? What’s the difference between these two seemingly ordinary people?
Let’s look at it from a more positive angle though. In asking this question, “Why am I myself, and not somebody else?” we create an opening for the answer: “Because I am myself, there is something only I can do.”