"Sam" behind their counter.
The supermarket owner is a purple pet with alien antennae, black hair, pink animal eyes and big black eyebrows. They are not visibly a boy or a girl, and thus their gender is not known. They sit behind their counter, partially hidden by the register, and seems to be watching the player's pet very carefully.
The name "Sam" sounds similar to the beginning of supermarket, and is gender neutral, but it is not mentioned in the game.