We need you to tell us your real last name, not a nick name or initial. This a matter if trust, identification and it's also needed for The Pawshake Guarantee.
Your last name will of course not be displayed on your profile or anywhere else. Only after a pet parent has booked your services and you have confirmed the booking will your last name be displayed to them (and vice versa), though ideally you already know such details about one another at that point.