The "front face" or "normal face" of a futakuchi-onna has +2 INT and +1 WIS, but -2 CON and -1 STR. If the "back face" or secondary mouth is allowed control of the body, these modifiers are flipped. The secondary mouth is also blind, so its actual attacks hit with disadvantage due to being unable to see.

The secondary mouth is prone to saying what the first is thinking. During certain social interactions (particularly ones in which the main body is lying or holding back her true opinions), 2d20 should be rolled -- one for the host and one for the second mouth. (These rolls can be done using specific skills, but can also default to CHA.) Whichever dice roll is higher is the one that gets to lead the interaction. If unsure whether or not to roll, consider this -- is your character hiding something or trying to be subtle? If yes, roll. Gain +1 in Persuasion and Deception.

Humans reach adulthood in their late teens and live less than a century.

Humans tend toward no particular alignment. The best and the worst are found among them.

Humans vary widely in height and build, from barely 5 feet to well over 6 feet tall. Regardless of your position in that range, your size is Medium.

Your base walking speed is 30 feet.

You can speak, read, and write Common and one extra language of your choice. Humans typically learn the languages of other peoples they deal with, including obscure dialects. They are fond of sprinkling their speech with words borrowed from other tongues: Orc curses, Elvish musical expressions, Dwarvish military phrases, and so on.