How would I make a StarterCharacter rare?

So, I have these exploding things in my game. Well, once in a while I want someone to spawn as a big version of this exploding thing. The method to spawn as a custom character I have put in is I put the exploding thing in StarterPlayer and name it StarterCharacter. I wondered about replacing both characters really fast, but what if someone managed to somehow slip through at the right moment, and become a big one too? I might be able to do it, but I want to know an efficient method. Thanks for any feedback.