I migrated an account from one Macbook to another (both Mountain Lion) using the account migration assistant. When starting the new macbook the account doesn't appear with the others on the login screen when starting up. I have to login to another account, logout and then it appears on the login screen. The login screen background also changes from white to textured grey/black background.
From OS X: About FileVault 2:
In order to enable the migrated user for FileVault:
Disable the "Automatic log in" xxxyyy" in Users and Groups.
Based on your updated answer, there must be two active login accounts than.
Also check the Account in question, for login shell setting. In case you do not know how to do this: In Users & groups, Right click on the xxxyyy account, opens advanced options showing the login shell type.
That might also explain different startup screen.
For your Information: Normally users in OSX have their shell defined as /bin/bash.
The OS login window will actually make sure people have a usable shell before showing them on the login screen.
For example: You can disable a user — revoking both their ability to login and their appearance in the login window user list — by changing the shell to /usr/bin/false.
Apple recomeds: If your computer starts up without displaying the login window, the computer is set up to log in to a specified user account automatically.
If a computer is shared by multiple users, it’s best to set up an individual account for each user and disable automatic login. Using separate accounts protects the information and settings of each user.
Choose Apple menu > System Preferences, click Users & Groups, and then click Login Options. If the options are dimmed, click the lock icon to unlock it, and then type an administrator name and password. Click Login Options, and then choose Off from the “Automatic login” pop-up menu. The next time you start up your computer, the login window appears, and a user name and password are required to log in.