The name used for the file structure on your machine is dependent on the e-mail address used as the userid for the Microsoft Account, not the full name you specify for yourself in the Microsoft Account.
If you use a Microsoft Account linked Win10 user account whatever name you have given yourself in that account will be displayed on the login/lock screen.
My name, in real life and used on my Microsoft Account is Brian Vogel. The e-mail address I used to establish that Microsoft Account starts with "britechguy@". The file structure for my user account on my machine is C:\Users\brite. I have another user account, also Microsoft Account linked, the first part of the e-mail address for that is "guyslp@". The file structure is C:\Users\guysl. It's clear that the file structure naming convention is to take the first 5 characters of the supplied e-mail address and use that for a given user's home folder.
You are not obligated to have both a First and Last name in your Microsoft Account, and I just went in to edit my profile and removed my last name just to test that out. Since you do not log in with either of those, but the e-mail address, they're simply allowed, not required. When I had neither first or last name entered in the profile what showed up on the login screen was the e-mail address, in full.
Addendum: The above presumes you created the account as a Microsoft Account linked user account from the start. If you convert a local account to Microsoft Account linked then your home folder will still stay the same as it was when you created the account as local.
Edited by britechguy, 15 April 2018 - 10:10 AM.
Brian AKA Bri the Tech Guy (website in my user profile) - Windows 10 Home, 64-Bit, Version 1809, Build 17763
Presenting the willfully ignorant with facts is the very definition of casting pearls before swine.
~ Brian Vogel