Which user account is always UID 0 on AIX?

The root account is the special user in the /etc/passwd file with the user ID (UID) of 0 and is commonly given the user name, root. It is not the user name that makes the root account so special, but the UID value of 0. This means that any user that has a UID of 0 also has the same privileges as the root user. Also, the root account is always authenticated by means of the local security files.

https://www.ibm.com/docs/en/aix/7.2?topic=passwords-root-account