This may be a little bit of a stretch, but I see it as a combination of two expressions: take a good look at something/someone and turn your back on something/someone.
We say "take a good look at [something/someone]" when we want the person to remember the thing or person being looked at. This is often used when the thing or person will be missing, removed, denied, or some similar state. For example, imagine a kidnapper taking a parent's children. The kidnapper might say
Take a good look at your kids! You won't be seeing anytime soon!
Figuratively, when we turn our backs on someone, we reject them:
turn your back on somebody/something
- To ignore, disregard, or exclude someone or something; to abandon, give up on, or forsake someone or something. It's clear that the company has turned its back on customers. After becoming a successful writer, Jim turned his back on all the people he used to know back home.
- COMMON If you turn your back on something, you reject it or stop being involved in it. He had turned his back on his Communist past and formed a completely new party. The organisation says that young people are increasingly turning their backs on marriage.
So, although X will literally see Charlie's back when Charlie skates away, I think it can also be interpreted figuratively. I would say it means something like remember me because I will reject you, or more plainly, remember our friendship because we are longer friends (since X stole Charlie's skateboard, according to Charlie).
I'm just guessing that the relationship is "friends", but it could be something else, like teammates, colleagues, or whatever the case may be in the text.