Formal/Neutral:
* Adeus, traquinas! (Goodbye, rascal!) - This is the most literal translation, but it can sound a bit too formal.
* Até logo, esperto! (See you later, smart one!) - This is a more informal and playful way to say goodbye.
Informal/Affectionate:
* Tchau, moleque! (Bye, kid!) - This is very informal and friendly.
* Até mais, safado! (See you later, you rascal!) - This is even more informal and playful, but it might be considered a little rude depending on the context.
* Vai lá, te cuida! (Go on, take care!) - This is a more general way to say goodbye and can be used in various contexts.
The best way to translate "bye rascal" into Portuguese is to consider the relationship between the speakers and choose an option that is appropriate for the situation.