the rhq_principal table stored username/password combinations. The rhq_subject table stores user information. You will find an "admin" user pre-defined in that table. You will need a different user name.
do you know if I can modify/delete this user? does it has any specific purpose?
Do not delete it, as it is used / required internally.
About renaming - I think this is a bad idea too.
You might want to raise a feature request in RHQ jira (http://jira.rhq-project.org/secure/Dashboard.jspa) to have this user renamed to something that is less likely to clash with commonly used names.
You could even supply a patch :-)))