The Arduino Leonardo board uses Serial1 to communicate via TTL (5V) serial on pins 0 (RX) and 1 (TX). Serial is reserved for USB CDC communication. For more information, refer to the Leonardo getting started page and hardware page.
The Pro Micro is basically a mini version of the Leonardo so the same way applies. You do not need an extra library as Serial/Serial1 is built into the Arduino core for that MCU.