This one goes in Stewy's head. It handles sensor input from the HC-SR04 and reads the joystick, then communicates that data to the base. It's the ESP32-C3 Super Mini, a single-core RISC-V board running at 160 MHz with Wi-Fi and Bluetooth 5 LE.

It's small enough to fit comfortably in the head housing and has a USB-C port for programming. Specs: 4MB flash, 400KB SRAM, 11 programmable GPIOs, 3.3V or 5V operation.