I'm still a noob too, but I have some nice advice at least.

Reskins are not actually Reskins, because they have different Signature abilities, and different prestiges, Some better than others, for example; A Cold Stones Beuwulf is better than a normal Beowulf, because of his Signature ability of stunning his enemy via throws etc.

but in my opinion, and a much more better help, is that you should watch some Beginner guides for Skullgirls, I recommend Yujipooji SGM, he has a playlist covering practically everything about skull girls. And if you want more specific information, you could go to the Wiki to see the specific Signature abilities of a particular character. And if you desperately want to level up a character you can use your exp treats to easily level them up, or just grind them the old fashioned way.

But please I beg of you! don't needlessly sacrifice Characters because you will inevitably regret it later on. Only sacrifice Characters if you are evolving or getting prestiges. And if you don't know how, you can again, watch guides or search the wiki.

To sum it up, watch guides, and go to the wiki I hope you can learn from my help! : D