The MCU's primary role in pairing the HC-05 slave module with a master Bluetooth device like the Mofii number pad is to facilitate the initial pairing process and potentially manage the HC-05's configuration, if needed. The HC-05, in its default slave mode, passively waits for a connection request from a master device. The MCU may not be directly involved in the Bluetooth pairing protocol itself, but it can enable the HC-05's AT command mode to change its configuration, if required. For example, the MCU can send commands to the HC-05 via a serial interface to configure its name, PIN code, or baud rate.