Your phone becomes the keyboard — stick, jump, abilities, weapons. You keep the mouse for aiming. Both devices must be on the same Wi-Fi. Nothing is stored anywhere and no server is involved: the two devices talk directly.
Click a key, then press the one you want. If that key is already taken the two swap places, so nothing is ever left unbound.
DESKTOP ONLY
This one wants a mouse and keyboard. Mouse look uses pointer lock, which phone and tablet browsers do not have.